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Electrochemical Energy Reviews

, Volume 2, Issue 4, pp 606–623 | Cite as

Voltage Decay in Layered Li-Rich Mn-Based Cathode Materials

  • Kun Zhang
  • Biao Li
  • Yuxuan Zuo
  • Jin Song
  • Huaifang Shang
  • Fanghua Ning
  • Dingguo XiaEmail author
Review article

Abstract

Compared with commercial Li-ion cathode materials (LiCoO2, LiFePO4, NMC111, etc.), Li-rich Mn-based cathode materials (LMR-NMCs) possess higher capacities of more than 250 mAh g−1 and have attracted great interest from researchers as promising candidates for long-endurance electric vehicles. However, unsolved problems need to be addressed before commercialization with one being voltage decay during cycling. Here, researchers have proposed that the mechanisms of voltage decay in Li-rich Mn-based cathode materials involve factors such as surface phase transformation, anion redox and oxygen release and have found evidence of transition metal-migration, microstructural defects caused by LMR and other phenomena using advanced characterization techniques. As a result, many studies have been conducted to resolve voltage decay in LMR-NMCs for practical application. Based on this, this article will systematically review the progress in the study of voltage decay mechanisms in LMR materials and provide suggestions for further research.

Graphic abstract

Keywords

Voltage decay Surface phase transformation TM migration Anion redox Microstructural defects Voltage hysteresis 

1 Introduction

Li-rich Mn-based cathode materials (LMR-NMCs), represented as xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, Mn), possess desirable characteristics such as high capacities greater than 250 mAh g−1 [1], low toxicity and low costs. As a result, these materials are considered to be promising cathode materials in high-energy-density lithium-ion batteries. However, LMR-NMCs also possess intrinsic shortcomings such as low initial Coulombic efficiencies, poor cycling stability, low rate performances and, especially, rapid voltage decay [2], making application in electric vehicles and grid energy storage devices difficult. Here, unlike electrochemical polarization that leads to increasing charge voltages and decreasing discharge voltages, voltage decay not only leads to continuously decreasing energy densities but also to difficultly in the charge management of battery management systems in EVs, further hindering commercialization. To address this, numerous studies have investigated the mechanisms and suppression methods of voltage decay, and as a result, this review will summarize these findings in terms of surface, bulk and geometric structures as well as particle morphology. In addition, possible correlations between voltage decay and voltage hysteresis are presented in this review along with proposals for the future research of Li-rich Mn-based cathode materials.

2 Voltage Decay Mechanisms

2.1 Crystal Structures of LMR-NMCs

Thackeray et al. [3] first synthesized Li1.09Mn0.91O2 through the delithiation of Li2MnO3 using an acid treatment in 1991, and based on this, the concept of xLi2MnO3·(1 − x) LiMnO2 was proposed. In general, LiMO2 possesses a 2D α-NaFeO2 layered structure with an R\( \overline{3} \)m space group in which similar to LiCoO2, Li and TM ions occupy the structure layer by layer. And in the case of Li2MnO3, which can be regarded as Li[Li1/3Mn2/3]O2, Li ions occupy a third of the sites in the TM layer to form a superstructure as evidenced by light-light-dark patterns under STEM [4], resulting in the transformation from a hexagonal crystal system to monoclinic symmetry with a C2/m space group. In general, LMR-NMCs can be thought of as a combination of two different crystal systems (Fig. 1) [5] in which the specific combination can be split into two types. Here, one combination results in the formation of a solid solution in the bulk area as observed by good correspondence between LMR-NMC XRD brag peaks and C2/m composites. For example, Jarvis et al. [4] observed the total superstructure area of Li[Li0.2Ni0.2Mn0.6]O2 using STEM and found through XRD that the structure of Li[Li0.2Ni0.2Mn0.6]O2 was either an R\( \overline{3} \)m with the partial combination of a C2/m space group or a C2/m space group with Ni ions partly arranged in the sites of Li and Mn that results in a short-ranged ordered structure. The researchers in this study also reported that their D-STEM results indicated no existence of the R\( \overline{3} \)m phase, thus classifying this material as a C2/m structure. The other combination of two different crystal systems lies in the nanoscale phase domain of the material, in which Yu et al. [6] proved the existence of phase domains using STEM, high-angle annular dark-field STEM and annular bright-field STEM in their study.
Fig. 1

Composition formation of an LMR-NMC structure.

Adapted with permission from [5]. Copyright 2015 Elsevier B.V

As for Li-Ni-Mn-O systems, researchers initially assumed a type of combination between two phases in which Thackeray et al. [7] depicted the LMR-NMC or the LMR-MC as ordered or partially disordered Li2MnO3 rather than a superlattice structure based on XRD refinement results. These researchers also proposed that the LMR-NMC was composed of Li2MnO3 and LiMO2 with Li2MnO3 playing a more significant role in the stabilization of the structure and the supplement of the Li content [1] and that the extent of TM layer disordering intensified as Co was added into the structure. In another study, Armstrong et al. [8] used neutron diffraction to investigate the structure of Li[Li0.2Ni0.2Mn0.6]O2 to report an R\( \overline{3} \)m phase with C2/m Li2MnO3 components. Alternatively, Lei et al. [9] demonstrated that Li[Li0.2Ni0.2Mn0.6]O2 did not possess long-range domains with clear boundaries as evidenced by EELS, EDS and HAADF-STEM and attributed this to the homogeneous Ni distribution in the bulk area being \( \leqslant \) 10 nm3. Abraham et al. [10] also analyzed HAADF-STEM and SAED results to report the lack of single monoclinic phases in Li[Li0.2Ni0.2Mn0.6]O2. And based on these results, there is a general consensus that solid solution structures possess possible local (short-ranged) binary phase structures, which guides further research into the chemical environments of oxygen or the behavior of transition metals during electrochemical processes in Li-rich cathode materials.

2.2 Oxygen Behavior and Anion Redox Mechanism

Because voltage decay is closely related to cycling, efforts have been made to investigate chemical structural changes during charge and discharge cycles. Here, researchers have reported that voltage decay is closely related to oxygen behaviors in both the bulk and the surface of LMR-NMCs in which at charging above 4.4 V, the Li2MnO3 structure (which was originally isolated) starts to activate. And to maintain charge balance (because no Mn valence oxidation is regarded), one Li+ escalation is equivalent to 1/4 oxygen (O2) escalation [11], resulting in a Li–O–Li configuration that can lead to oxygen vacancies in the bulk due to oxygen release, possibly promoting structure transformation and TM migration [12]. In addition, Meng et al. [13] correlated TEM/STEM, EELS, Synchrotron X-ray diffraction and micro-strain effects to explain the effects of oxygen vacancy on transition metals and structural changes and demonstrated that oxygen vacancies can facilitate TM migration by decreasing migration free energy [14]. Aside from oxygen loss at the surface of LMR-NMCs [15], oxygen behaviors in the bulk areas have also been investigated, in which reversible oxygen redox with conversion between O2−/O22− or O2−/O2n with 3 > n >1 has been proposed [16, 17, 18]. For example, Tarascon et al. [19] attributed the highly reversible capacity of Li-rich cathode materials to the reactions of O2−/O22− or O2−/O2n with 3 > n > 1 in the bulk using XPS and EPR spectra. These researchers also provided visible proof of O-O dimers in Li-rich cathode materials using HAADF-STEM in high-covalent Li2IrO3 systems [20]. Furthermore, Zhou et al. [21] used operando Raman spectra to directly observe reversible oxygen behaviors and Delmas et al. [22] synthesized Li1.20Mn0.54Co0.13Ni0.13O2 using the sol-gel method, which resulted in a 2-phase emergence on the high-voltage plateau for up to 100 cycles. Here, the researchers attributed this to different surface oxygen loss and bulk oxygen recharge reactions. Delmas et al. also investigated and proved the reversible oxygen redox process. [23] Another cause of voltage decay lies in the oxygen holes during the charge process, in which Bruce et al. [24, 25] found that oxygen can transform into oxygen holes as generated on O 2p orbits during the extraction of Li ions rather than the emergence of peroxide- and superoxide-like species. Overall, a general integration of bulk and surface oxygen redox behaviors is needed to reveal the overall voltage decay mechanism [26, 27].

Various studies have explored the physical and chemical origins of the oxygen redox in TM-O systems of LMR-NMCs and traditional lithium cathode materials and have reported that despite inevitable oxygen redox mechanisms at high voltages due to physical and chemical properties, effective measurements can be taken to minimize this based on oxygen redox theories (discussed below in part 2). For example, Ceder et al. [28] investigated the structural and chemical properties of layered and disordered Li-excess materials using rational DFT calculations and band structure analyses and revealed that the origin of high capacity as created by oxygen redox can be credited to the energy overlap between unhybridized oxygen bonds and TM bonds (the Li–O–Li configuration, Fig. 2). These researchers also reported that diverse metal and oxygen ligands resulted in distinct oxygen behaviors in which non-TMs were inclined to form peroxo-like O–O bonds with more flexible bond rotations and that the competition between oxygen and TM ions was a result of the energy band of TM orbitals, in which V, Cr, Mo or Mn orbitals could theoretically be maximized in the redox process. In another study, Xia et al. [29] used XPS, XAS, EXAFS and DFT calculations as well as charge-density difference maps to reveal the requirements for oxygen redox in Li-excess systems, in which the introduction of Fe into Ti-based Li-excess materials can trigger oxygen redox similar to Ti-based sulfide materials with the introduction of Co. And due to the vague explanations of the conductivity theory, these researchers developed a U-Δ theory that correlated well with experimental findings in which U represented d-d Coulombic and exchange interactions, whereas Δ represented electron fluctuations involving O 2p bands. Here, the researchers reported that if UΔ, electrons tended to be active on d orbitals only, whereas if UΔ oxygen redox will be triggered due to lower energy gaps. These researchers also reported that their theory of the oxygen charging process involved heavier elements such as Ru, Mn, Co, Ni and Fe presenting larger U and easier oxygen redox, whereas lighter metals such as Ti, Nb, Ta, V and Zr presented tendencies to deactivate. Therefore, deeper studies into anion redox mechanisms, structure transformations and ion migrations during variable state of charge (SOC) are key to understanding voltage decay mechanisms.
Fig. 2

a Li–O–Li configuration in Li-excess oxides. b Schematic diagram of the energy band overlap within O and TMs.

Adapted with permission from [28]. Copyright 2016 Macmillan Publishers Limited

2.3 Phase Transformation and Microstructural Defects

In a further study, Meng et al. [30] compared pristine and post-cycling Li[Li1/5Ni1/5Mn3/5]O2 using HAADF-STEM to investigate structure transformations and reported that after ten cycles, spinel structures clearly appeared on the surface of Li[Li1/5Ni1/5Mn3/5]O2 that were not present in the pristine sample, revealing the structural instability of Li[Li1/5Ni1/5Mn3/5]O2 during cycling and a surface-originated transformation (Fig. 3). In addition, EELS spectra obtained in this study revealed a Mn-reduction from + 4 to + 3 on the surface of Li[Li1/5Ni1/5Mn3/5]O2 along with the weakening of Mn–O bonds during cycling in which quantitative analysis suggested that Li loss exceeded 5% on the surface, further verifying TM migration into the Li layer and the significance of the stabilization of LMR-NMC surface structures.
Fig. 3

HAADF-STEM images of a a pristine Li[Li1/5Ni1/5Mn3/5]O2 sample with a well-layered honeycomb structure, and b an electrochemically cycled Li[Li1/5Ni1/5Mn3/5]O2 sample with obvious differences from the surface to the bulk, in which TM ions have migrated into Li layers in the surface area. Adapted with permission from [30].

Copyright the Royal Society of Chemistry 2011

In terms of phase change distinction between Li2MnO3 and LiMO2 (nanoscale phase domains), Wang et al. [31] used HAADF-STEM to observe the coexistence of R\( \overline{3} \)m and C2/m space groups as well as phase change during cycling and reported that for R\( \overline{3} \)m (LiMO2), TM ions migrated to Li layers directly to form a quasi-LiMn2O4 spinel phase, whereas C2/m (Li2MnO3) tended to be amorphous and form disordered-directed spinel structures due to the loss of O and structural degradation. As a result, the researchers suggested the feasibility and effectiveness of surface phase change suppression in LiMO2 through coating or doping (i.e., AlF3 coatings) and the ineffectiveness for C2/m Li2MnO3 due to inevitable oxygen loss during the electrochemical process (Fig. 4).
Fig. 4

Three phases of the original material along with different routes of TM migration as a result of the differences in R\( \overline{3} \)m and C2/m phases.

Reprinted with permission from [31]. Copyright 2012 American Chemical Society

Layered-to-spinel transformations in LMR-NMCs are common occurrences contributing to voltage decay. However, XRD and TEM patterns cannot precisely characterize structures such as standard LiMn2O4, and charging curves lack a ~ 4 V reduction peak that is characteristic of Li ions migrating from tetrahedral sites in standard spinel structures. Therefore, Wang et al. [32] purposed that the LMR-NMC surface phase variation is a quasi-spinel structure. Here, the researchers studied LMR-NMC structural change after one cycle and discovered the continuous appearance of the R\( \overline{3} \)m group due to the activation of Li2MnO3 as well as small quantities of TM-ion migration to Li layers that are spread from the surface to the bulk area. These researchers reported that this TM-ion migration was not continuous enough to transform into standard LiMn2O4, but rather to a LT-LiCoO2-defected spinel structure in which after ten cycles, there were still Li sites ~ 2 nm from the surface that were not occupied by TM ions. In addition, these researchers also reported that with further charge-discharge, the structure can ultimately transform into a disordered rock salt type with the Fm\( \overline{3} \)m space group, which was further verified by fast Fourier transform. (Fig. 5a shows the integral path of surface change in LMR-NMCs.)
Fig. 5

a Layered-quasi-spinel-disordered rock salt transformation on the surface of LMR-NMCs. Adapted with permission from [32]. Copyright 2015 American Chemical Society. b Mechanistic diagram of Mn and Ni ion migration differences upon cycling and its relation to phase transformation. Adapted with permission from [33]. Copyright 2013 American Chemical Society

Researchers are also interested in the role of each element in the phase transformation process of LMR-NMCs because of their different chemical environments. For example, Boulineau et al. [33] revealed the migration features of LMR-NMCs during cycling in which EELS showed that the Ni content on the surface of LMR-NMCs increased, whereas Mn quantity decreased in which the researchers suggested that due to the appearance of unstable MO5, Mn ions should migrate into Li vacancies parallel to the surface and cause bulk densification or contraction and spinel phase transformation. In addition, these researchers reported that because the velocity of Ni ion migration was slower than Mn ion migration (more Li ions surrounded Mn ions), Ni ions segregated on the surface of LMR-NMCs, leading to a quasi-rock salt structure (Fig. 5b), providing the first evidence of Mn and Ni segregation in LMR-NMCs.

Overall, preceding studies have systematically concluded that the phase transformation in LMR-NMCs from layered to quasi-spinel structures and subsequently to disordered rock salt structures is closely related to TM-ion migration in which Ceder et al. [34] suggested the meta-stability of TM ions in octahedral sites and that TM ions such as Ti4+ tended to be stable in tetrahedral sites. In addition, Debasish et al. [35] used in situ XRD to demonstrate that Mn ions migrated from TM layer octahedral sites (OTM) to Li layer tetrahedral sites (Tli) and subsequently to Li layer octahedral sites (Oli). Despite these findings however, LMR-NMCs are composites with multiple elements and complex structures, and therefore, it is difficult to systematically investigate the migration of inner ions and the type of ions migrating. To address this, Tarascon et al. [36] used Li2RuO3 as a model LMR-NMC due to its analogous structure and superior performance as compared with Li2MnO3 and through the doping of Ti4+\Sn4+ and characterizations with HAADF-STEM and XPS were able to clearly verify TM-ion migration including TM migration in Tli and Oli in which the researchers took the anion redox mechanism into consideration to calculate the energy barriers of TM migration, with results being consistent with experimental results. Here, the researchers reported that during cycling, TM ions (mostly Ti4+) can become trapped in Tli sites, whereas for Sn4+ and Ru4+ this was minor. In addition, these researchers also reported that the largest voltage decay was found in Ti-doped cathodes. In the same study, they innovatively considered the anion redox mechanism to calculate the energy barrier for TM migration, whose results kept coherence with experimental consequences. Tarascon et al. [36] also addressed the importance of ionic radii on ion migration based on Ti4+ migration and purposed that ion migration can be suppressed through doping with larger and more electroactive ions such as Nb and Mo.

Hu et al. [2] also detected nanoscale pore formation during the synthesis of LMR-NMCs and reported that such surface microdefects can further exacerbate oxygen release and phase transformation as induced by ion migration, increasing the growth of pores. This group in a subsequent study was also able to observe these microdefects on LMR-NMCs during cycling through 3D electron tomography [37] (Fig. 6) and reported that the growth of these microdefects on pristine LMR-NMCs was closely related to intrinsic characteristics in which the direct cause of voltage decay in their LMR-NMCs was decreasing Ni (+ 3/+ 4) contributions and increasing Mn (+ 3/+ 4) and Co (+ 2/+ 3) contributions at lower voltage plateaus beyond 83 cycles.
Fig. 6

3D electron tomography of the pristine structure pattern with volume renditions, cross-sectional progress and the pattern after 15 cycles showing the occurrence of obvious defects on the surface and the bulk.

Adapted with permission from [37]. Copyright 2018, Springer Nature

2.4 Voltage Decay Mechanism in LMR-NMCs

In general, the voltage decay mechanism of LMR-NMCs involves several aspects in which during charging, LMR-NMC materials with oxygen redox release surface oxygen to promote the reduction in surface ions and therefore trigger reactions at lower voltage plateaus. This release of oxygen will subsequently generate oxygen vacancies, which will weaken bonds between TMs and O, allowing for easier TM migration to Li sites and phase transformations. In addition, reversible reactions in the bulk can also enable oxygen to affect the LMR-NMC total structure and promote TM migration and the conversion of the bulk based on strain, crystalline structure and the placement of different facets. Furthermore, nanoscale microdefects on LMR-NMC materials can expand the surface-specific area to allow for more oxygen to escape, leading to more intensive phase transformations. The pores of LMR-NMCs as a result of microdefects can also facilitate oxygen release through the enlargement of escape pathways and surface structure changes. Moreover, phase changes can further exacerbate structural degradation and oxygen release as well as the enlargement of small pores.

Overall, because the voltage decay of LMR-NMCs is closely related to surface transformations, surface modification to control oxygen activation and prevent structure degradation is the most direct method to resolve this issue. However, because the reason for voltage decay is multi-faceted, with oxygen redox being a major reason, methods to mitigate or even eliminate voltage decay need to take on a holistic approach in which all three primary reasons, including surface modification, bulk structure and synthesis conditions, are considered. Here, surface modification is a basic tool that can stabilize surface structures to atomically stabilize oxygen redox centers and avoid electrolyte erosion to protect cathode materials. As for the control of bulk structures, this can allow for the control of the intrinsic activation of oxygen redox and the stability or reversibility of structures. In addition, mesoscale improvements are also needed based on research on microstructural defects. And in the following sections, a summary will be presented of the solutions that have been proposed in the literature in response to voltage decay in LMR-NMCs based on these three aspects (Fig. 7).
Fig. 7

The relationship between oxygen redox, phase transformation and microstructural defects and their relationships with voltage decay

3 Solutions to Resolve Voltage Decay

3.1 Surface Modification Methods to Alleviate Voltage Decay

Based on the study of voltage decay mechanisms in LMR-NMCs, researchers have reported that surface structure stability has a significant impact on voltage decay, and as a result, surface modifications of chemical structures have been widely applied. For example, Zheng et al. [38] improved the irreversible capacity loss of LMR-NMCs through coating with TiO2 and AlF3 and Manthiram et al. [39] subsequently discovered that Al2O3 and AlPO4 were the most effective materials to alleviate irreversible capacity loss in LMR-NMCs through doping out of several materials including Al2O3, CeO2, ZrO2, SiO2, ZnO and AlPO4 in which the researchers purposed that surface coatings can maintain oxygen vacancies generated through anion redox. In addition, Zhao et al. [40] coated a layer of MnO2 onto the surface of Li[Li0.2Ni0.2Mn0.6]O2 using co-precipitation and reported that the prepared cell demonstrated a capacity retention rate of more than 97% under 0.2 C after 50 cycles and that their XPS results indicated that as Li intercalated in the material, a LixMnyO2 structure formed. Furthermore, He et al. [41] created a cladding of amorphous V2O5 on the surface of LMR-NMCs and compared its electrochemical performance to the same LMR-NMC doped with V2O5 in the bulk area. Here, the researchers reported that the surface-doped material possessed a more stable surface structure and more resistance to surface ion transfer, whereas the bulk-doped material merely acted as Li intercalation sites. And although these studies mainly focused on capacity loss in the electrochemical process, attention to voltage decay is also needed urgently. And with the progress of voltage decay mechanisms, researchers are beginning to concentrate on the effects of surface coating on voltage decay. For example, Zheng et al. [32, 42] selected AlF3 (possessing excellent effects after being coated) to study the influence of surface-doped fluorine on voltage decay and reported that the AlF3 coating can diminish phase transformation (from layered to quasi-spinel structure) and stabilize the transformation structure (Fig. 8), in which STEM indicated an obvious alleviation of surface phase change after coating. Here, the researchers suggested that AlF3 can act as an oxygen buffer layer to reduce the activation of oxygen reduction in which the analysis of the Mn L3/L2 value in EELS revealed a higher average covalence (3.89 for Mn) than that of the non-coated sample (3.58 for Mn), proving that surface coating can decrease the generation of reduced elements on the surface of LMR-NMCs to reduce TM-ion migration and voltage drops. Furthermore, these researchers also reported that fluoride coatings can mitigate surface corrosion caused by fluorides in electrolytes to suppress the exacerbation of surface microstructural defects.
Fig. 8

Comparison between AlF3-coated and uncoated LMR-NMC materials based on cycling stability and voltage decay.

Adapted with permission from [42]. Copyright 2014 American Chemical Society

In another study, Shang et al. [43] coated a RuO2 layer onto a Rux-LMR-NMC using co-precipitation to obtain an 8-μm 3D spherical structure and reported that the resulting structure demonstrated a 98% capacity retention rate after 100 cycles with nearly no voltage decay (Fig. 9), in which larger-scale versions of the resulting particles can achieve higher tap density. Liu et al. [44] also coated LMR-NMCs with a 1.5 nm layer of Nb2O5 that tended to enter the Li layer as revealed by DFT calculations and reported that their STEM results were consistent with theoretical conclusions in which stronger Nb–O bonding can weaken the energy overlap between TM3d and O to stabilize the surface oxygen structure. Here, the researchers reported that better control of phase transformation in the coated LMR-NMC was observed based on results from STEM, Raman spectra and O K-edge spectra in which after coating, voltage decay decreased to 136 mV between the sixth and the 100th cycles with a large gap of 593 mV as compared with the uncoated sample. Similar results were also obtained with TiO2- and ZrO2-coated materials in the same work, suggesting that coating with heavier atom structures is an effective method to reduce hybridization between TMs and O and stabilize surface anion atoms [45].
Fig. 9

Cycling performances of the Rux -LMR-NMC samples: a cycling performances under 1 C; b rate capabilities of the samples; c discharge profiles under normalized capacity; d dQ/dV versus V plots for the Ru0.03 sample.

Adapted with permission from [43]. Copyright 2018 American Chemical Society

Apart from coating LMR-NMC surfaces with metal oxides [43, 44, 46], fluorides [42, 47] or phosphates [48, 49, 50, 51], other methods have also been proposed to achieve surface stabilization. For example, Qiu et al. [52] designed a 20 nm layer of oxygen vacancies using a gas-solid interfacial reaction with NH4HCO3 as a CO2 source and observed through HAADF-STEM XPS, FTIR and STEM-EELS spectra that after modification, the LMR-NMC structure remained undamaged and that through DEMS and EELS, the surface oxygen vacancies can facilitate ion transfer and prevent structural transformation and oxygen release during charge and discharge in which capacity retention of 300 mAh g−1 after 100 cycles and an initial Coulombic efficiency of 93.2% were obtained, which was better as compared with pristine LMR-NMCs. Here, the researchers attributed the reduced voltage decay to the enhanced stability of the resulting surface structure in which cycling profiles demonstrated a decrease in surface oxygen reactivity and an increase in the transformation of bulk oxygen to oxides without turning into O2, which can damage surface oxygen reactions and reduce structural stability. Wu et al. [53] also created oxygen deficiencies through the coating of a layer of c-PAN on LMR-NMC surfaces and reported similar results. In another study, Ning et al. [54] constructed a layer of V6-type defective graphene onto the surface of LMR-NMCs to enhance thermal stability and reported through theoretical calculations that the V6-type defective graphene-coated LMR-NMC possessed enlarged ΔG (Gibbs free energy) for oxygen release, which can reduce the release of surface oxygen that was thermodynamically and dynamically favorable on uncoated pristine LMR-NMCs. In addition, the researchers reported that the coating of LMR-NMCs with V6-type defective graphene changed the migration energy barrier of Mn ions from 0.9 eV (uncoated) to ~2.0 eV (coated), reducing Mn migration and improving voltage retention rates in terms of oxygen loss and TM migration in which at an operating condition of 2.0–4.8 V, the coated material only displayed 0.35 V voltage decay after 100 cycles as compared with 0.53 V and 0.68 V for the uncoated and completely coated (coated completely with graphene without defects) samples. In addition, under an operating condition of 2.0–4.6 V, the V6-type defective graphene-coated LMR-NMC only demonstrated voltage decay of 0.08 V after 100 cycles as compared with 0.61 V and 0.47 V for the uncoated and completely coated samples. Furthermore, Yang et al. [55] developed a surface potassium-doped LMR-NMC cathode material with significantly reduced voltage decay and minor voltage drops after 100 cycles and reported that surface metal doping can not only restrain structural change but also stabilize the chemical environments of surface O atoms. Yan et al. [56] also used PEG as a surfactant and dispersant to achieve the uniform dispersal of Mn and Ni to reduce crystalline defects and oxygen release, and Zheng et al. [57] proposed that surface modifications through the coating of inactive compounds can sacrifice energy density and rate capability and, therefore, suggested LiFePO4 as an ideal coating material to mitigate voltage fading, in which experimental results showed minor voltage decay in LiFePO4-doped samples as compared with uncoated samples. Interestingly, the researchers in this study synthesized their LiFePO4-doped samples using the sol-gel method in which citric acid was used to treat the surface of pristine materials to allow for the well fitting of LiFePO4 into the surface-layered structure. Moreover, Li et al. [58] reported that the coating of a layer of 1% Li2SnO3 with a spinel heterostructure on to LMR-NMCs can stabilize surface structures.

3.2 Tuning of Bulk Electronic and Crystalline Structures to Repress Voltage Decay

3.2.1 Reducing Oxygen Redox and Stabilizing Redox Center

Because anion redox reactions can enhance TM migration and surface structural change, it is important to stabilize anion structures during the charge and discharge process. And aside from surface modification, researchers have conducted significant research into the tuning of bulk electronic structures to alter the electronic state and crystalline structure of LMR-NMCs to address anion redox reactions in which one effective method is to reduce oxygen reactions and stabilize anion redox centers. For example, Li et al. [59] doped polyanions (BO3)3− and (BO4)5− into LMR-NMCs to decrease the energy level of O2p due to stronger binding energy between polyanions and oxygen and weaken the bond between O and TM ions (Fig. 10) in which DFT calculations suggested that polyanions can enter the tetrahedral and triangle sites to elongate TM–O bonds and intensify the distortion of local MnO6 structures. In addition, weakened TM–O bonds can also stabilize oxygen structures, reduce oxygen release and relieve structure collapse. And as a result, EXAFS results in this study indicated that the Co–O chemical environment became stabler in different lithiated stages. Furthermore, similar polyanion doping strategies were also reported in the literature [60, 61].
Fig. 10

Schematic of the charge compensation process in which a decrease of ~ 0.8 eV in the O 2p band can be observed after polyanion doping.

Adapted with permission from [59]. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Manthiram et al. [62] also prepared Li1.2Mn0.6−xRuxNi0.2O2 materials through the doping of Ru4+ into Li2MnO3 and reported that with increase in the Ru content, charging voltage plateaus continuously dropped and that at x\( \geqslant \) 0.4, the structure became monoclinic P12/m with Ru-Ru dimers splitting the Ru4+ t2g orbital into a bonding orbital and an antibonding orbital, which decreased the covalence between Ru–O bonds and lessened the participation of O in electrochemical reactions. Moreover, the doping of large-scale atoms such as Zr [63, 64], Sn [65], K [66], Cr [67, 68], Se [69], Fe [70] and La [71] to stabilize layered structures has also been reported in the literature with similar results.

Anions aside from oxygen can also be doped into LMR-NMC structures to mitigate voltage decay. For example, Li et al. [72] doped F- ions into the bulk area of LMR-NMCs to reduce oxygen redox and phase transformation during the oxidation-redox process in which the participation of F- can enlarge the bulk volume and crystal cell parameters due to the partial reduction in TM ions and alleviate the erosion of electrolytes. And despite some capacity loss in the resulting structure, stability increased greatly, suggesting that bulk electron structure tuning is promising in the stabilization of anion redox centers to reduce voltage decay. Yan et al. [73] also proposed the doping of Cl ions into Li1.11Ni0.89O2−yCly (y\( \leqslant \) 0.22) through theoretical calculations and reported that Cl can effectively decrease the overlap between Ni and O and stabilize the oxygen redox center. In addition, these researchers also reported that the Cl-doped cathodes showed significantly reduced average charge voltages as compared with F-doped materials, thus avoiding electrolyte decomposition at high voltages. Moreover, the doping of Cl was also found to be able to meliorate rate properties in which TDOS indicated an increase in electron conductivity and higher defect formation energy to suppress the formation of anti-structural defects. Despite these promising results however, more experiments need to be conducted to further verify these theoretical results and discover new methods.

3.2.2 Control of TM Distribution

Another method to mitigate voltage decay is based on bulk element distributions that rely on TM behaviors other than oxygen redox reactions. This is because the mobility of TM migration is dependent on the distinction between Mn and Ni and the accompanying elemental segregation. As a result, researchers have made efforts to tailor the distribution of TMs in integral materials in which concentration-gradient structures with a low surface Ni content have been purposed [74, 75]. Wang et al. [76] also compared LMR-NMC synthesis methods including co-precipitation (CP), sol-gel (SG) and hydrothermal-assisted (HA) methods in the control of TM distribution and reported that the HA method resulted in materials with less-activated Ni2+ at the pristine stage and better rate capabilities at rates higher than 1C as compared with materials obtained through CP and SG methods. More importantly, these researchers reported that voltage decay of the samples obtained through HA over 200 cycles was only 0.15 V as compared with 0.25 V for the CP and SG samples, in which HAADF-STEM indicated an obvious decrease in the coexistence of the two phases and an increase in the C2/m domain of the HA sample. And although the segregation of Ni ions in the CP and SG samples was also clearly observed in the EDS mapping results, all results indicated a more uniform distribution of Ni in the HA sample, which enhanced the stability of Mn valence and suppressed surface side reactions to alleviate voltage decay, demonstrating the importance of synthesis methods in industrial production. In another study, Wang et al. [77] achieved the synchronous formation of a low surface Ni content and a surface spinel structure on LMR-NMCs through an initial treatment of carbonate precursors with ammonia followed by calcination with Li2CO3. As a result, the resulting material displayed better rate capabilities due to a 3D spinel channel [53, 78, 79, 80, 81] and a more stable surface structure with decreased voltage decay and capacity loss, again demonstrating the importance of optimal synthesis strategies to control the distribution of TMs to suppress surface instability. Furthermore, to understand the intrinsic driving force of voltage decay with regard to cation distribution, Myeong et al. [82] compared the reversibility of cations (oxygen-active Mn and inactive Ni or Co) and the covalence between O and TMs between well-ordered LMR-NMCs and long-range disordered, short-range ordered LMR-NMCs using operando XANES and differentiated XANES spectra and reported that the formation of well-ordered MO4 structures during cycling can rearrange and trigger cation migration, whereas disordered structures can reduce voltage decay, demonstrating the feasibility of Li-rich cathode materials with partially disordered structures. And overall, through the simple manipulation of synthesis methods, stabler Li-rich materials that are feasible in real industrial productions can be realized.

3.2.3 Macroscopic Understanding of Oxygen Behavior

With respect to overall oxygen behaviors and the macroscopic impact on the bulk of LMR-NMCs, Singer et al. [83] used in situ Bragg coherent diffractive imaging to observe dislocation nucleation during high-voltage charging (Fig. 11) and reported that this type of structural dislocation correlated with oxygen redox and rearrangement and can occur after Li-ion diffusion or TM migration to induce unfavorable planes for Li on TM layers and increase the bulk Gibbs free energy, leading to bulk area differentiation and voltage decay without notable changes in morphology. To address this nucleation dislocation and recover the average voltage, the researchers annealed the cycled sample and reported an average voltage as high as the pristine sample. Here, the researchers suggested that annealing can thermodynamically rearrange the location of atoms into a more ordered system and can be applied in industrial practice. Aside from structural transformations involving atom migration and rearrangement, the influence of charge-discharge cycles on the large-scale dimension of LMR-NMCs also needs to be explored and samples at different SOCs need to be studied on a macroscopic scale. And in practical terms, the exploration of dislocation at the particle scale is closely related to microscopic transformations.
Fig. 11

Formation process of nucleation dislocation networks upon charging.

Adapted with permission from [83]. Copyright 2018 Springer Nature

3.3 Morphology Management for Better Performance

In addition to crystalline size and grain boundaries, the performance of LMR-NMC-based batteries is also dependent on surface cracks and defects, which can decrease structural stability. For example, Hu et al. [2] manipulated the defect morphology of Li2Ru0.5Mn0.5O3 by discharging the corresponding cell to 1 V to intercalate excess Li to allow Li2Ru0.5Mn0.5O3 to form a 1T structure with TMs in the octahedral sites and with excess Li in the tetrahedral sites. As a result, volume enlargement and microstructural cracks were observed in which the defective material demonstrated worsened voltage decay as compared with the unmodified material. In addition, partial distribution function results indicated similar short-range normality in the 1 V discharged and normally charged materials but a difference in bulk morphology as confirmed by STEM, with XAS data showing increased oxygen release and reduction in Mn ions in both materials, leading to accelerated structural degradation and voltage decay. However, these cracks and microdefects are features of large surface area nanoparticles and prevent LMR-NMCs from becoming smaller in size. In addition, researchers have used novel transmission X-ray microscopy to detect changes in cathode particles such as strain and complexity during different cycling processes on the mesoscale (10 μm) [84], and Cho et al. [85] coated Co and Li (can diffuse into primary particles) onto the primary particles (the units to assemble a secondary particles) of a Ni-rich cathode to mitigate the evolution of microdefects and reported that this method can be possibly used in LMR-NMC materials with modifications.

Voltage decay is caused by surface transformation, which in turn is caused by nanoscale materials with large surface areas. This large surface area allows for the uniform distribution of particles and enhanced ion transfer, but is also detrimental to structural stability because structural transformation and oxygen release occur mostly on the surface. Therefore, the design of novel structures without apparent kinetic disadvantages is required. For example, Oh et al. [86] synthesized an LMR-NMC with 10 μm secondary particles using hydrazine hydrate measurements with flake-shaped nanoscale or sub-microscale particles inside the secondary particles and reported that this type of morphology can protect inner particles from the corrosion of electrolytes and maintain rate capabilities through the control of inner particles. Similarly, Zhang et al. [87] designed a spherical hierarchical material with outstanding electrochemical stability and Li et al. [88] created fusi-form hierarchical micro- and nano-Li1.2Ni0.2Mn0.6O2 accompanied with a (110) orientation crystal plane that is beneficial for ion diffusion (Fig. 12). Furthermore, Luo et al. [89] studied the influence of active and inactive exposed facets based on electrochemical performance and reported that active facets can allow for higher capacity, better rate capability and lower voltage hysteresis than inactive materials, but resulted in worsened voltage decay, suggesting that stabler structures with faster Li diffusion need to be achieved for better electrochemical performances. He et al. [90] also reported a 3D porous Li-rich material with stable average voltages of ~ 3.5 V after 100 cycles, and Zhang et al. [91] designed a hollow porous bowl-shaped Li-rich cathode material with a larger surface area and reported minor voltage decay with excellent rate capabilities. And based on these results, the protection of LMR-NMC surfaces that does not sacrifice nanoscale advantages is a promising method in which the enhancement of LMR-NMC morphology is the protection of its surface and the stability of the crystalline structure from a macroscopic perspective to surface and bulk structures.
Fig. 12

Mechanism and principle of the suppression of voltage decay.

Adapted with permission from [88]. Copyright the Royal Society of Chemistry 2016

3.4 Influence of Geometric Structure on Voltage Decay

Traditionally, LMR-NMCs are O3-type structures with ABCABC oxygen packing. And despite huge capacities due to the activation of anion redox, capacity loss and voltage decay are serious issues hindering commercialization. Alternatively, Thackeray et al. [92] reported a promising novel O2-structured LMR-NMC with ABAB oxygen packing in 1999 that was stabler than O3-type LMR-NMCs but possessed a reversible capacity of only 150 mAh g−1. In addition, Zuo et al. [93] converted the P2−-type precursor NaxLi0.25M0.75O2 (M = Mn0.675Co0.325) into O2-type Li1.25Co0.25Mn0.5O2 through molten salt exchange (Fig. 13) and reported that after 50 cycles, the average voltage dropped by only 0.07 V, which was almost negligible compared with the voltage decay of O3-type LMR-NMCs. Furthermore, the O2-type structure showed the reduced shifting of the reduction peak to lower voltages during cycling in dQ/dV curves and in situ XRD suggested little variation to the superstructure peaks in the O2-type structure, whereas the O3-type structure showed the prominent weakening of superstructure peaks due to structural rearrangements. Moreover, DFT calculations revealed that after delithiation, the O–O bond length in the O2-type structure was still 2.44 Å, whereas in the O3-type structure, it condensed to 1.38 Å, which was close to O2. And overall, both the experimental and the theoretical results demonstrate the stability of the O2-type structure, suggesting that further research into O2-structured LMR-NMCs, corresponding mechanisms and large-scale productions is needed, especially the oxygen redox compensation mechanism that is different from O3 Li-rich materials and is a promising pathway to obtain various O2-structured LMR-NMCs.
Fig. 13

a Cycling performances of an O2-type Li-rich cathode. b Average voltage profile of an O2-type cathode between the first cycle and the 50th cycle. Adapted with permission from [93]. Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Correlation Between Voltage Decay and Voltage Hysteresis

Despite tremendous progress in the suppression of voltage decay, capacity decline and low rate capacity in LMR-NMCs, voltage hysteresis remains elusive and unresolved (Fig. 14), in which Tarascon et al. [94] reported a ~ 1 V voltage postponement through dQ/dV as compared with the voltage profile of LMR-NMCs, with an average 4.2 V charging peak reducing to ~ 3.25 V in the discharge stage within the same cycle. This phenomenon is dependent on both dynamic (sluggish kinetics) and thermodynamic origins, in which the former is related to both ion diffusion and electron transfer with possible phase change and the latter suggests a charge-discharge voltage gap even if currents reach 0 (e.g., testing the battery under < 0.02 C) or under GITT/PITT (galvanostatic intermittent titration technique/potentiostatic intermittent titration technique) experiments (simulating a thermodynamic equilibrium state) and can drastically reduce the energy efficiency of LMR-NMC devices. In addition, Dreyer et al. [95] suggested that minor thermodynamic voltage hysteresis in LiFePO4 systems was due to charge-discharge stages particle by particle, representing a multiparticle system through mathematical analysis. However, the cause of voltage hysteresis in LMR-NMC systems is not similar to LiFePO4 systems, in which the electrochemical process of LMR-NMC systems involves complex oxygen redox reactions. Based on this, researchers have invented window tests to explore the voltage hysteresis of LMR-NMCs at different SOCs, during which researchers charge and discharge the battery to obtain distinct voltage windows to detect gradual growth of voltage hysteresis and the turning point for the significant increase in voltage gap [96, 97]. Here, experimental results showed strong correlations between oxygen redox and voltage hysteresis, which were asymmetric as a result of the path-dependent process. Furthermore, oxygen redox can lead to cation migration, and as a result, researchers are trying to precisely probe structural changes during cycling at key points of voltage hysteresis. Moreover, nuclear magnetic resonance methods have been utilized [98] to detect delicate changes in Li sites during charge-discharge, revealing that even minor structural defects can have huge impacts on voltage profiles, thus revealing a connection among anion redox, TM migration and voltage hysteresis. Voltage decay and voltage hysteresis are also connected because both are a result of structural degradation during electrochemical processes [99]. And despite the findings in these studies, the definitive reason for the hysteretic feature of oxygen redox reactions in LMR-NMCs remains vague; and therefore, studies should focus on the qualitative characterization of this phenomenon and, to a limited extent, quantitative characterizations in which efforts should be taken to either providing well-grounded explanations for the origin of voltage hysteresis or to quantitatively characterizing the open-circuit voltage gap. Here, it is important to note that LMR-NMC systems are complex systems with multiple cations coupled with oxygen redox, making clear research difficult. In addition, there are currently no powerful measurements that can avoid the effects of such characteristics and new measurement methods are needed. Therefore, future studies should focus on the origin of voltage hysteresis both theoretically and practically.
Fig. 14

dQ/dV curve for an LR-NMC material with oxygen redox hysteresis marked in yellow.

Adapted with permission from [96]. Copyright 2013 American Chemical Society

5 Conclusion and Perspectives

Voltage decay in layered Li-rich Mn-based cathode materials is closely associated with electrochemical reactions, structural changes and intrinsic properties during electrochemical reactions. In addition, voltage decay relies on different cutting voltages, charging rates and the intrinsic properties of materials, making the quantitative summary of improvements in voltage decay difficult. Therefore, the proper evaluation of voltage decay can be done based on charging situations (the voltage range, current, temperature, etc.), voltage decay after specific cycles after capacity normalization or average voltage profiles under cycles and tendencies in open-circuit voltage excluding polarization. And overall, this review has related oxygen release with oxygen reversible redox as led by anion redox with structural change and microstructural defects and illustrated their effects on each other to trigger voltage decay. Based on this, future research should take into consideration these three aspects. Here, surface modifications and the control of bulk electrons and elements can effectively stabilize the chemical structure of LMR-NMCs [100], in which the inherent defects of the nanoscale materials necessitate the reformation of synthesis methods, with successful attempts to alter nanoscale materials into new morphologic materials being conducted. In addition, the emergence of novel geometric configurations for LMR-NMCs has provided new avenues of research to resolve voltage decay problems. Here, surface modification is the most effective and direct method to suppress voltage decay. As for the control of bulk structures through cation and anion doping, polyanion doping is a rational and efficient method accompanied with sufficient theoretical explanations. Furthermore, morphological and geometrical transformations are also promising methods to reduce voltage decay in LMR-NMCs and thus require further research. Interestingly in all cases, methods to mitigate voltage decay will also lead to performance improvements as a whole, including capacity retention rates and rate capabilities, and can be possibly attributed to the stabilization of surface and bulk structures, thus encouraging the comprehensive improvement in LMR-NMCs. However, unsolved issues related to voltage hysteresis need to be further explored both kinetically and thermodynamically. Therefore, continuous research is required to explore more advanced modifications in the industrial application of LMR-NMCs and to obtain optimal combinations of different methods to reduce voltage decay and other issues in LMR-NMCs. More importantly, advanced characterization techniques need to be applied to elucidate the driving forces behind LMR-NMCs to obtain practical LMR-NMC materials with optimal properties and to combine fundamental research with industrial production to allow for commercialization.

Notes

Acknowledgements

This work was supported by the Beijing Municipal Natural Science Foundation (No. 2181001), the National Natural Science Foundation of China (Nos. 51671004 and U1764255) and the National Key Research and Development Program (2016YFB0100200). All sources of support for this work are gratefully acknowledged.

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Copyright information

© Shanghai University and Periodicals Agency of Shanghai University 2019

Authors and Affiliations

  1. 1.Beijing Key Laboratory of Theory and Technology for Advanced Batteries MaterialsCollege of Engineering, Peking UniversityBeijingChina
  2. 2.Beijing Innovation Center for Engineering Science and Advanced TechnologyPeking UniversityBeijingChina

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