Surface Structural Transition Induced by Gradient Polyanion‐Doping in Li‐Rich Layered Oxides: Implications for Enhanced Electrochemical Performance

Abstract

To meet the demanding requirements in plug-in hybrid electric vehicles (PHEVs) or electric vehicles (EVs), higher energy density materials, such as the Li-rich, layered manganese-based oxides (LLOs) with the general formula xLi 2 MnO 3 ·(1-x)LiTMO 2 (TM = Mn, Ni, Co, etc.), are promising candidates as they possess higher reversible capacity (>250 mAh g −1 ), improved safety and much reduced cost. [4][5][6][7][8][9] Recent microscopic evidence reveals the intergrowth of rhombohedral LiTMO 2 (R-3m) and the monoclinic Li 2 MnO 3 -like layered structure (C/2m) at the atomic scale in the oxide grains. [10] The Li 2 MnO 3 component serves as an electrochemically active phase for Li storage when cycled above 4.5 V versus Li/Li + . [8,[11][12][13][14] Nevertheless, these LLO materials undergo steady voltage/capacity decay when cycled above 4.5 V, resulting in a substantial decrease of the cathode energy density. [15][16][17][18] The origin of voltage/capacity decay upon cycling stems from cation migration between TM layers and Li layers and subsequent phase transformation. [19,20] The cationic doping with other metallic cations (such as Mg, [21] Al, [22] Ti, [23] Sn, [24] Ru, [25] Y, [26] Zn, [27] etc.) and polyanion doping based on nonmetal elements, such as BO 4 5− , [28] SiO 4 4− , [29] PO 4 3-, [30] etc., have been employed to improve the cyclic durability by weakening the TM-O covalency in the oxygen closepacked structure. In addition, surface coatings using metal oxides, [31][32][33][34] fluorides and phosphates, [35][36][37] LiNiPO 4 and Li 3 VO 4 , [38][39][40] have been applied to protect the surface structure from side reactions with the electrolyte under high voltage and to restrain the layered-to-spinel transformation which occurs preferentially on the crystal surface and leads to capacity fading of LLO materials. However, the ionic dopants and coating materials are mostly electrochemically inactive, so the improved cycling stability is achieved at the expense of reduced specific capacity/energy density of the cathode. Moreover, a conformal and continuous coating on the surface of oxide particles is rather difficult to obtain practically. Hence, advancing the structural and cycling stability in both the bulk material and the surface structure through a simple way is highly desired for potential applications of LLO materials.Herein, we develop a novel LLO material with a nanoscaled spinel-like surface layer through gradient doping of polyanions Surface Structural Transition Induced by Gradient Polyanion-Doping in Li-Rich Layered Oxides: Implications for Enhanced Electrochemical PerformanceYing Zhao, Jiatu Liu, Shuangbao Wang, Ran Ji, Qingbing Xia, Zhengping Ding, Weifeng Wei,* Yong Liu, Peng Wang,* and Douglas G. Ivey Lithium-rich layered oxides (LLOs) exhibit great potential as high-capacity cathode materials for lithium-ion batteries, but usually suffer from capacity/ voltage fade during electrochemical cycling. Herein, a gradient polyaniondoping strategy is developed to initiate surface structural trans...

Feasibility of Cathode Surface Coating Technology for High‐Energy Lithium‐ion and Beyond‐Lithium‐ion Batteries

“…In general, the pristine forms of cathode materials suffer from phase transitions, metallic ion dissolution into the electrolyte, and mechanical stress, enabling particle microcracks (initiated from the particle's surface), over the course of electrochemical cycling. Considering the various breakthroughs in half-cell and full-cell LIBs based on surface engineered cathode materials, [7,15,36,37] we believe surface coating is one of the most effective research strategies, which can be extended to beyond-lithium-ion technologies to mitigate possible cathode phase transitions, microcracks (including intragranular crack formation at high cut-off voltage), and electrolyte decomposition; and can be used to achieve high-energy and high-voltage rechargeable batteries, which could be viable over a wide temperature range and with high charge cut-off voltages. Therefore, various research strategies, such as tuning the morphology of the cathode material, doping with foreign metallic elements, using electrolyte additives to enable stable and precise solid electrolyte interphase (SEI) layer formation, etc., have been employed to improve the overall electrochemical performance of rechargeable batteries.…”

“…[45][46][47][48] Therefore, the efficient usage of oxygen atoms for prolonging the safety life of cathode materials and promoting high energy density could be a potential future research challenge. [37] The high-resolution transmission electron microscope (HR-TEM) image ( Figure 2b) of surface coated-LRO (Li 1.17 Mn 0.5 Ni 0.17 Co 0.16 (PO 4 ) x O 2−4x , 5% P@LLO) shows the formation of a mixed spinel-like and layered phase on the surface of the particle, which is evident from fast Fourier transform (FFT) electron diffraction patterns of selected selected regions 1 and 2 in Figure 2b (Figure 2c). This means that particle degradation from the surface needs to be controlled to achieve prolonged battery performance under practical conditions.…”

“…The cell capacity is proportionate to the amount of facile Na-ion intercalation and de-intercalation of the host material (anode or cathode). 2017, 29, 1605807 www.advancedsciencenews.com www.advmat.de [37] (6 of 12) 1605807 revealed significant undesirable phase transitions at a high charge cut-off voltage, which are responsible for capacity degradation over cycling. Due to such inherent properties, various research attempts have been devoted to cell potential to enhance the overall energy density of SIBs.…”

Retarding Phase Transformation During Cycling in a Lithium‐ and Manganese‐Rich Cathode Material by Optimizing Synthesis Conditions

“…To stabilize the surface and bulk phase structure of LLOs, many efforts have been made, including ionic doping, [23][24][25] surface coating, [26][27][28] and morphology design. [29][30][31] However, the procedures in these treatments are complicated and have limited improvement.…”

Uniform Na+ Doping‐Induced Defects in Li‐ and Mn‐Rich Cathodes for High‐Performance Lithium‐Ion Batteries

“…[5,6] Despite the attraction in energy density, there are still many problems need to be solved before their commercialization. [17] Alkali metal elements like K, [18][19][20] Na, [21,22] alk-earth metal elements Mg [23][24][25][26] and transition elements including Fe, [27,28] Cr, [29] Ti, [30] and La, [31] had been used to optimize the ionic diffusion coefficient and enhance the structure stability. [11][12][13][14][15][16] Doping is considered as an effective strategy to improve the performance of the LMR cathode materials.…”