High Cut-off Voltage Research Articles

Electrochemical tailoring MXene terminal to elevate operation voltage of Mo2CTx-protected current collector over 5 V

Elevating operation voltage of lithium ion battery (LIB) is inevitably restricted by the corrosion of cathode aluminum (Al) current collector. In this report, MXene (Mo2CTx) is proposed as an artificial passivation layer on Al surface. To broaden the electrochemical stability of Mo2CTx, a mild tactic of potentiostatic polarization is applied to tailor Mo2CTx terminal at around 5.3 V vs. Li/Li+ for 60 s. After electrochemical tailoring, the -F terminal ratio is significantly increased. The obtained Mo2CFx exhibits uniform electrical conductivity similar to bare Al and outstanding antioxidation over 5 V vs. Li/Li+. As a consequence, when Mo2CFx is requisitioned to protect Al current collector, a high-voltage LIB equipped with LiCo1/3Ni1/3Mn1/3O2 (NMC333) in LiPF6 electrolyte manifests an overall enhanced electrochemical performance under a high cut-off voltage of 4.6 V. The discharge capacity retention is increased from 7.0 % to 62.3 % after 500 cycles at 0.5C. And the rate performance at 3C is also improved by about 300 %. This work contributes to the surface engineering and application of MXene nanosheets, as well as provides an effective strategy for developing stable high-voltage LIBs.

Improvement of electrochemical performance of LiNi0.5Co0.2Mn0.3O2 by LaF3 coating at high cut-off voltage

Improvement of electrochemical performance of LiNi0.5Co0.2Mn0.3O2 by LaF3 coating at high cut-off voltage

High-Voltage Layered Manganese-Based Oxide Cathode with Excellent Rate Capability Enabled by K/F Co-doping

P-type layered manganese-based materials are prone to undergo lattice oxygen oxidation accompanied by oxygen layer slipping and even unfavorable phase transitions at around 4.2 V, giving rise to rapid discharge capacity decline and inferior structural stability, which restricts their operating voltage and energy density. Here we propose a modification strategy for layered Na0.67MnO2 material with K/F co-doping so as to reach the optimization of lattice oxygen oxidation behavior and structural stability at a high cut-off voltage of 4.4 V. Combining X-ray powder diffraction refinement results, ex situ X-ray photoelectron spectrometry analysis, high-resolution transmission electron microscopy, and electrochemical characterization, our work demonstrates that an appropriate amount of K/F co-doping has a favorable effect on the formation of well-reversible lattice oxygen oxidation behavior and the improvement of Na layer spacing. Owing to the synergetic effect of potassium and fluorine atoms, the initial discharge capacity is up to 210.2 mA h g–1 at 0.1 C and 140.2 mA h g–1 at 1 C with 73% capacity retention after 100 cycles and high discharge capacity of 115.0 mA h g–1 at 5 C, with 68.1 mA h g–1 retained after 200 cycles. The kinetic analysis shows that the optimal K0.05Na0.62MnO1.95F0.05 sample exhibits the largest sodium ion diffusion coefficient and the smallest electrochemical polarization, which paves a novel path for the application of layered manganese-based oxides in high-voltage cathode materials for sodium-ion batteries.

Tri-sites co-doping: An efficient strategy towards the realization of 4.6V-LiCoO2 with cyclic stability

Lithium cobalt oxide (LCO) is extremely attractive for the volumetric and gravimetric energy density at high cut-off voltage, but the degradation issues we are always grappling with becomes much tougher. Here, an unconventional strategy of multi-sites co-doping is proposed using cheap and abundant elements like Na, Si, Al, Fe and F. By summarizing the fading behaviors of all the samples, the capacity fading shows a linear correlation with the doping elements. The optimized co-doping strategy enables each doping elements to achieve the full potential in the promotion of cyclic stability. The resultant LCO maintains a capacity retention of 81.8% after 500 cycles at 4.6 V of the full battery. Further evidences signify that the capacity fading in the early stage is mainly caused by surficial side reactions and the accompanying structural damage, while the fading in the later stage is due to particle cracking generated by the repeated planar gliding and the anabatic side reaction of the newly exposed surface. Therefore, not only the main factor of structural damage caused by surface side reactions should be considered, but more attention need to be paid on the planar gliding when designing more stable LCO. The tri-sites co-doping strategy, which can better integrate the inhibitory effect of different doping sites on planar gliding, provides a new idea for further improving the cyclic stability of high-voltage LCO.

Mechanical densification synthesis of single-crystalline Ni-rich cathode for high-energy lithium-ion batteries

The intergranular microcracking in polycrystalline Ni-rich cathode particle is led by anisotropic volume change and stress corrosion along grain boundary, accelerating battery performance decay. Herein, we have suggested a simple but advanced solid-state method that ensures both uniform transition metal distribution and single-crystalline morphology for Ni-rich cathode synthesis without sophisticated co-precipitation. Pelletization-assisted mechanical densification (PAMD) process on solid-state precursor mixture enables the dynamic mass transfer through the increased solid-solid contact area which facilitates the grain growth during sintering process, readily forming micro-sized single-crystalline particle. Furthermore, the improved chemical reactivity by a combination of capillary effect and vacancy-assisted diffusion provides homogeneous element distribution within each primary particle. As a result, single-crystalline Ni-rich cathode with PAMD process has eliminated a potential evolution of intergranular cracking, thus achieving superior energy retention capability of 85% over 150 cycles compared to polycrystalline Ni-rich particle even after high-pressure calendering process (corresponding to electrode density of ∼3.6 g cm−3) and high cut-off voltage cycling. This work provides a concrete perspective on developing facile synthetic route of micron-sized single-crystalline Ni-rich cathode materials for high energy density lithium-ion batteries (LIBs).

Film-forming mechanism of 1,3-propanediolcyclic sulfate as a bifunctional additive for 4.45 V graphite/LiCoO2 batteries

Increasing the cutoff voltage is efficient to increase the energy density of LiCoO2-based lithium-ion batteries to meet consumers' demand for electronic devices. However, higher cut-off voltage may lead to continuous decomposition of electrolyte and shorten the battery's cycle life. Herein, 1,3-propanediolcyclic sulfate (PCS) is designed as a film-forming additive to improve the electrochemical performance of high-voltage graphite/LiCoO2 cells. The combined density functional theory (DFT) calculations and experimental results indicate that PCS participates in the film formation on both LiCoO2 cathode and graphite anode side to regulate the interfacial reaction and inhibit the corresponding gas production. Thus, the capacity retention of 4.45 V graphite/LiCoO2 pouch cells cycled at 45 °C after 300 cycles is elevated from 59.5% to 80.1% by incorporating 1.0 wt% PCS additive into the base electrolyte. It is found that PCS follows the C-O bond breaking mechanism on the positive electrode and S-O bond breaking mechanism on the negative electrode, respectively. In addition, PCS may undergo H-transfer reaction at the cathode in the first cycle because the -O-SO2- group appears as oxidation product in ex-situ X-ray photoelectron spectroscopy (XPS) results. This finding provides new insights for understanding the mechanism of PCS acting as a high-voltage additive.

Corrigendum to “Review of spinel LiMn2O4 cathode materials under high cut-off voltage in lithium-ion batteries: Challenges and strategies” [J. Electroanal. Chem. 920 (2022) 116623

Corrigendum to “Review of spinel LiMn2O4 cathode materials under high cut-off voltage in lithium-ion batteries: Challenges and strategies” [J. Electroanal. Chem. 920 (2022) 116623

Insights into the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries.

Lithium-based rechargeable batteries have dominated the energy storage field and attracted considerable research interest due to their excellent electrochemical performance. As indispensable and ubiquitous components, electrolytes play a pivotal role in not only transporting lithium ions, but also expanding the electrochemical stable potential window, suppressing the side reactions, and manipulating the redox mechanism, all of which are closely associated with the behavior of solvation chemistry in electrolytes. Thus, comprehensively understanding the solvation chemistry in electrolytes is of significant importance. Here we critically reviewed the development of electrolytes in various lithium-based rechargeable batteries including lithium-metal batteries (LMBs), nonaqueous lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs), and aqueous lithium-ion batteries (ALIBs), and emphasized the effects of interactions between cations, anions, and solvents on solvation chemistry, and functions of solvation chemistry in different types of electrolytes (strong solvating electrolytes, moderate solvating electrolytes, and weak solvating electrolytes) on the electrochemical performance and redox mechanism in the abovementioned rechargeable batteries. Specifically, the significant effects of solvation chemistry on the stability of electrode-electrolyte interphases, suppression of lithium dendrites in LMBs, inhibition of the co-intercalation of solvents in LIBs, improvement of anodic stability at high cut-off voltages in LMBs, LIBs and ALIBs, regulation of redox pathways in LSBs and LOBs, and inhibition of hydrogen/oxygen evolution reactions in LOBs are thoroughly summarized. Finally, the review concludes with a prospective outlook, where practical issues of electrolytes, advanced in situ/operando techniques to illustrate the mechanism of solvation chemistry, and advanced theoretical calculation and simulation techniques such as "material knowledge informed machine learning" and "artificial intelligence (AI) + big data" driven strategies for high-performance electrolytes have been proposed.

Understanding of working mechanism of lithium difluoro(oxalato) borate in Li||NCM85 battery with enhanced cyclic stability

Despite the significant advances achieved in recent years, the development of efficient electrolyte additives to mitigate the performance degradation during long-term cycling of high-energy density lithium||nickel-rich (Li||Ni-rich) batteries remains a significant challenge. To achieve a rational design of electrolytes and avoid unnecessary waste of resources due to trial and error, it is crucial to have a comprehensive understanding of the underlying mechanism of key electrolyte components, including salts, solvents, and additives. Herein, we present the utilization of lithium difluoro(oxalate) borate (B) (LiDFOB), a B-containing lithium salt, as a functional additive for Li||LiNi0.85Co0.1Mn0.05O2 (NCM85) batteries, and comprehensively investigate its mechanism of action towards enhancing the stability of both anode and cathode interfaces. The preferential reduction and oxidation decomposition of DFOB- leads to the formation of a robust and highly electronically insulating boron-rich interfacial film on the surface of both the Li anode and NCM85 cathode. This film effectively suppresses the consumption of active lithium and the severe decomposition of the electrolyte. Furthermore, the presence of B elements in the cathode-electrolyte interfacial film, such as BF3, BF2OH, and BF2OBF2 compounds, can coordinate with the lattice oxygen of the cathode, forming strong coordination bonds. This can significantly alleviate lattice oxygen loss and mitigate detrimental structural degradation of the Ni-rich cathode. Consequently, the Li||NCM85 battery cycled in LiDFOB-containing electrolyte displays superior capacity retention of 74% after 300 cycles, even at a high charge cut-off voltage of 4.6 V. The comprehensive analysis of the working mechanisms of LiDFOB offers valuable insights for the rational design of electrolytes featuring multifunctional lithium salts or additives for high energy density lithium metal batteries.

Magneto-electrochemistry driven ultralong-life Zn-VS2 aqueous zinc-ion batteries.

The development of high energy density and long cycle lifespan aqueous zinc ion batteries is hindered by the limited cathode materials and serious zinc dendrite growth. In this work, a defect-rich VS2 cathode material is manufactured by in situ electrochemical defect engineering under high charge cut-off voltage. Owing to the rich abundant vacancies and lattice distortion in the ab plane, the tailored VS2 can unlock the transport path of Zn2+ along the c-axis, enabling 3D Zn2+ transport along both the ab plane and c-axis, and reduce the electrostatic interaction between VS2 and zinc ions, thus achieving excellent rate capability (332 mA h g-1 and 227.8 mA h g-1 at 1 A g-1 and 20 A g-1, respectively). The thermally favorable intercalation and 3D rapid transport of Zn2+ in the defect-rich VS2 are verified by multiple ex situ characterizations and density functional theory (DFT) calculations. However, the long cycling stability of the Zn-VS2 battery is still unsatisfactory due to the Zn dendrite issue. It can be found that the introduction of an external magnetic field enables changing the movement of Zn2+, suppressing the growth of zinc dendrites, and resulting in enhanced cycling stability from about 90 to 600 h in the Zn||Zn symmetric cell. As a result, a high-performance Zn-VS2 full cell is realized by operating under a weak magnetic field, which shows an ultralong cycle lifespan with a capacity of 126 mA h g-1 after 7400 cycles at 5 A g-1, and delivers the highest energy density of 304.7 W h kg-1 and maximum power density of 17.8 kW kg-1.

Origin and characterization of the oxygen loss phenomenon in the layered oxide cathodes of Li-ion batteries.

Li-ion batteries have been widely applied in the field of energy storage due to their high energy density and environment friendliness. Owing to their high capacity of ∼200 mA h g-1 and high cutoff voltage of ∼4.6 V vs. Li+/Li, layered lithium transition metal oxides (LLMOs) stand out among the numerous cathode materials. However, the oxygen loss of LLMO cathodes during cycling hampers the further development LLMO cathode-based Li-ion batteries by inducing a dramatic decay of electrochemical performance and safety issues. In this regard, the oxygen loss phenomenon of LLMO cathodes has attracted attention, and extensive efforts have been devoted to investigating the origins of oxygen loss in LLMO cathodes by various characterization methods. In this review, a comprehensive overview of the main causes of oxygen loss is presented, including the state of charge, side reactions with electrolytes, and the thermal instability of LLMO cathodes. The characterization methods used in the scope are introduced and summarized based on their functional principles. It is hoped that the review can inspire a deeper consideration of the utilization of characterization techniques in detecting the oxygen loss of LLMO cathodes, paving a new pathway for developing advanced LLMO cathodes with better cycling stability and practical capabilities.

Bulk oxygen release inducing cyclic strain domains in Ni-rich ternary cathode materials

Nickel-rich layered transition metal oxide is limited by the poor structural stability during cycling as cathode materials for next-generation lithium-based automotive batteries. In the past, the poor electrochemical performance was mainly attributed to cracks and formation of rock-salt phase on the particle surface at high potentials. Rarely is the effect of bulk phase structure evolution on properties discussed. Here, we report a bulk oxygen release induced dynamic accumulative electrochemical–mechanical coupling failure mechanism. Domain-like rock salt phases are generated due to the oxygen release and transition metals migration in the bulk region of LiNi0.6Co0.2Mn0.2O2 (NCM622) particles at the first cycle high cutoff voltage. Then, reversible compressive/tensile lattice strain alternately dominate around the domain boundary and accumulate with cycling, leading to capacity fading and becoming the origin of intracrystalline cracks. The results suggest that, in addition to the side effects from the surface, the structural transformation of the bulk plays an important role in the capacity fading. The stabilization of lattice oxygen in bulk region is a feasible solution to suppress the structural transition and the inhomogeneous stress distribution.

Enhanced Electrochemical Performance of LiNi1/3Co1/3Mn1/3O2 at a High Cut-Off Voltage of 4.6 V by Li1.3Al0.3Ti1.7(PO4)3 Coating

At present, LiNi1/3Co1/3Mn1/3O2 (NCM) is a widely used material in the commercial market due to the easy control of the preparation process and usage environment. However, its capacity keeps fading when the cut-off voltage increases. In this research, an Li1.3Al0.3Ti1.7(PO4)3 (LATP) coating method is proposed to improve the cycle performance of LiNi1/3Co1/3Mn1/3O2 at a high cut-off voltage of 4.6 V. The battery prepared with LATP-modified NCM exhibits an increased discharge capacity retention of 92.37% after 100 cycles at 0.2C (1C = 200 mA g−1), while the bare NCM only presents 64.28%. Our results indicate that LATP-surface coating might be a useful method to increase the cycle stability of NCM and other high-capacity cathode materials.

Progress of Single-Crystal Nickel-Cobalt-Manganese Cathode Research

The booming electric vehicle industry continues to place higher requirements on power batteries related to economic-cost, power density and safety. The positive electrode materials play an important role in the energy storage performance of the battery. The nickel-rich NCM (LiNixCoyMnzO2 with x + y + z = 1) materials have received increasing attention due to their high energy density, which can satisfy the demand of commercial-grade power batteries. Prominently, single-crystal nickel-rich electrodes with s unique micron-scale single-crystal structure possess excellent electrochemical and mechanical performance, even when tested at high rates, high cut-off voltages and high temperatures. In this review, we outline in brief the characteristics, problems faced and countermeasures of nickel-rich NCM materials. Then the distinguishing features and main synthesis methods of single-crystal nickel-rich NCM materials are summarized. Some existing issues and modification methods are also discussed in detail, especially the optimization strategies under harsh conditions. Finally, an outlook on the future development of single-crystal nickel-rich materials is provided. This work is expected to provide some reference for research on single-crystal nickel-rich ternary materials with high energy density, high safety levels, long-life, and their contribution to sustainable development.

Understanding the failure mechanism towards developing high-voltage single-crystal Ni-rich Co-free cathodes

Benefited from its high process feasibility and controllable costs, binary-metal layered structured LiNi0.8Mn0.2O2 (NM) can effectively alleviate the cobalt supply crisis under the surge of global electric vehicles (EVs) sales, which is considered as the most promising next-generation cathode material for lithium-ion batteries (LIBs). However, the lack of deep understanding on the failure mechanism of NM has seriously hindered its application, especially under the harsh condition of high-voltage without sacrifices of reversible capacity. Herein, single-crystal LiNi0.8Mn0.2O2 is selected and compared with traditional LiNi0.8Co0.1Mn0.1O2 (NCM), mainly focusing on the failure mechanism of Co-free cathode and illuminating the significant effect of Co element on the Li/Ni antisite defect and dynamic characteristic. Specifically, the presence of high Li/Ni antisite defect in NM cathode easily results in the extremely dramatic H2/H3 phase transition, which exacerbates the distortion of the lattice, mechanical strain changes and exhibits poor electrochemical performance, especially under the high cutoff voltage. Furthermore, the reaction kinetic of NM is impaired due to the absence of Co element, especially at the single-crystal architecture. Whereas, the negative influence of Li/Ni antisite defect is controllable at low current densities, owing to the attenuated polarization. Notably, Co-free NM can exhibit better safety performance than that of NCM cathode. These findings are beneficial for understanding the fundamental reaction mechanism of single-crystal Ni-rich Co-free cathode materials, providing new insights and great encouragements to design and develop the next generation of LIBs with low-cost and high-safety performances.

In situ mitigating cation mixing of Ni-rich cathode at high voltage via Li2MnO3 injection

Ni-rich layered oxides (LiNixCoyMnzO2, x ≥ 0.8) have been under intense investigation as cathode materials for high-energy rechargeable lithium ion batteries (LIBs) due to their high capacity and relatively low cost. However, Ni/Li cation mixing, in most cases, brings about capacity degradation, structure evolution and poor thermal stability, especially at high cut-off voltage. Herein, a universal strategy with novel mechanism-in situ mitigating cation mixing at 4.55 V via injecting Li2MnO3 has been achieved (label as LD-NCM811), significantly improving the electrochemical property, structural integrity and thermal stability of Ni-rich cathode materials compared with the conventional NCM811. LD-NCM811 maintains a high capacity retention of 93% at 0.3 C after 200 cycles at 25 °C with negligible voltage decay of 40 mV, whereas the NCM811 shows a retention of 68% and large voltage decay of 248 mV, and the corresponding cation mixing has been mitigated from 13.5% to 7.5%. At the temperature of 45 °C, LD-NCM811 still keeps a considerable capacity retention of 93% at 1 C, significantly superior to the NCM811 with 75%. Characterization and calculation reveal that the excellent performances result from the Li2MnO3 phase with unique superlattice providing lithium voids in transition metal (TM) oxide layers when it is charged above 4.5 V, which is favorable for the mixed Ni ions migrating back to TM layers instead of blocking the lithium channel. This new finding establishes a general strategy for mitigating cation mixing of NCM811 to realize its application in high energy density and safety batteries.

A bi-functional strategy involving surface coating and subsurface gradient co-doping for enhanced cycle stability of LiCoO2 at 4.6 V

Layered LiCoO2 (LCO) acts as a dominant cathode material for lithium-ion batteries (LIBs) in 3C products because of its high compacted density and volumetric energy density. Although improving the high cut-off voltage is an effective strategy to increase its capacity, such behavior would trigger rapid capacity decay due to the surface or/and structure degradation. Herein, we propose a bi-functional surface strategy involving constructing a robust spinel-like phase coating layer with great integrity and compatibility to LiCoO2 and modulating crystal lattice by anion and cation gradient co-doping at the subsurface. As a result, the modified LiCoO2 (AFM-LCO) shows a capacity retention of 80.9% after 500 cycles between 3.0 and 4.6 V. The Al, F, Mg enriched spinel-like phase coating layer serves as a robust physical barrier to effectively inhibit the undesired side reactions between the electrolyte and the cathode. Meanwhile, the Al, F, Mg gradient co-doping significantly enhances the surficial structure stability, suppresses Co dissolution and oxygen release, providing a stable path for Li-ions mobility all through the long-term cycles. Thus, the surface bi-functional strategy is an effective method to synergistically improve the electrochemical performances of LCO at a high cut-off voltage of 4.6 V.

Nonflammable all-fluorinated electrolytes enabling high-power and long-life LiNi0.5Mn1.5O4/Li4Ti5O12 lithium-ion batteries

LiNi0.5Mn1.5O4 (LNMO)/Li4Ti5O12 (LTO) spinel-spinel batteries have appealing features of high energy, high power and inherent safety. However, cycling high-voltage LNMO cathodes causes severe oxidation of conventional carbonate-based electrolytes and leads to extensive capacity decay. Herein, we report that a nonflammable all-fluorinated electrolyte can support high-rate and inherent-safe 3.2 V LNMO/LTO batteries. The nanoscale fluorinated interphase stabilizes cathodic structure and suppresses side reactions during cycling, even at a high cutoff voltage of 5.0 V. The LNMO/Li cell in the all-fluorinated electrolyte delivers superior cyclability with 90.8 % capacity retention at 1 C over 1000 cycles. The LNMO/LTO cell exhibits great practical potential with capacity retention greater than 93.0 % over 1500 cycles at 5 C. In addition, the all-fluorinated electrolyte allows the LNMO/LTO cells to operate over wide temperatures. This work highlights a facile method for realizing the commercialization of LNMO/LTO lithium-ion batteries.

Ni-rich cathode material with isolated porous layer hindering crack propagation under 4.5 V high cut-off voltage cycling

This study reports on a preparation of novel Ni-rich cathode material with isolated porous layer and its application into cathode in lithium ion batteries extending cut-off potential into 4.5 V. Using intermittent addition of Al salt during co-precipitation of NixCo1-x(OH)2 (0 < x < 1), the isolated porous layer is easily formed inside the final active material particles maintaining overall mechanical integrity and their spherical shape without the change of external particle morphology. Even under severe cut-off potential of 4.5 V, volume expansion/contraction by anisotropic mechanical stress from hexagonal 2 to 3 (H2 → H3) phase transition is greatly reduced by hindering inner particle crack propagation into the external area at particle surface. Due to this phenomena, 227 mAh/g initial capacity at 0.1C was obtained upon 86 mol% of Ni content, with 75 % cathode capacity retention after 100 cycles under 1C, which is highest available capacity value among ever reported. Under full cell configuration, capacity retention at 100 cycles was 77 % and total available energy was increased into 15 % when compared with normal cycling condition of 4.3 V cut-off. From microtomic transmission electron microscope analysis, the porosity of inner particle was maintained as low as 6.67 % under 12 % increase of average primary particle size even after 100 cycles under 4.5 V cut-off voltage. From our advanced utilization of capacity and cut-off voltage, available energy in full cell application of our Ni-rich cathode material exhibited 20 % increase when compared with 4.3 V cut-off voltage.

Unraveling the degradation mechanism of LiNi0.8Co0.1Mn0.1O2 at the high cut-off voltage for lithium ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM811) layered oxides have been regarded as promising alternative cathodes for the next generation of high-energy lithium ion batteries (LIBs) due to high discharge capacities and energy densities at high operation voltage. However, the capacity fading under high operation voltage still restricts the practical application. Herein, thecapacity degradationmechanismof NCM811 at atomic-scale isstudied in detail under various cut-off voltages using aberration-corrected scanning transmission electron microscopy (STEM).It is observed that the crystal structure of NCM811 evolution from a layered structure to a rock-salt phase is directly accompanied by serious intergranular cracks under 4.9 V, which is distinguished from the generally accepted structure evolution of layered, disordered layered, defect rock salt and rock salt phases, also observed under 4.3 and 4.7 V. The electron energy loss spectroscopy analysis also confirms the reduction of Ni and Co from the surface to the bulk, not the previously reported only Li/Ni interlayer mixing. The degradation mechanism of NCM811 at a high cut-off voltage of 4.9 V is attributed to the formation of intergranular cracks induced by defects, the direct formation of the rock salt phase, and the accompanied reduction of Ni2+ and Co2+ phases from the surface to the bulk.