Extending the Service Life of High‐Ni Layered Oxides by Tuning the Electrode–Electrolyte Interphase

Abstract

organic carbonate electrolytes used. [2][3][4][5] Specifically, the parasitic electrolyte decomposition occurs on the charged cathode surface, resulting in the formation of cathode-electrolyte interphase (CEI) with complicated surface chemistry. [6][7][8][9][10][11] Moreover, the surface degradation on cathode usually involves active mass dissolution, which is a common phenomenon in various cathodes. [12] Subsequently, the dissolved transition-metal ions reach the anode side through chemical crossover and poison the anode-electrolyte interphase (AEI) on graphite surface. [13,14] Thus, understanding the chemical and electrochemical reaction between the electrode and electrolyte, which influences the composition, microstructure, and chemical properties of the formed electrolyteelectrolyte interphases (EEIs), is crucial to establishing high-energy-density Li-ion batteries with long, stable service life.Conventionally, inert materials coating (e.g., Al 2 O 3 , [15] AlPO 4 , [16] AlF 3 , [17] etc.) has been applied to modify the cathode surface chemistry and mitigate the interphase degradation on the cathode. Nevertheless, the cathode bulk particle could gradually lose contact with the coating layer due to the anisotropic volume change during lithiation and delithiation. [18,19] On the other hand, electrolyte additive, which is able to regulate the EEI chemistry through an in situ modification strategy, has been widely studied and proved to ensure high Coulombic efficiency and voltage efficiencies with long cycle life. [20][21][22][23] However, due to the complexity and air-sensitivity of the EEI chemistry, there is a limited understanding of the EEI configuration and architecture, and how different components influence the EEI layer properties and battery performance is yet to be fully understood. [24][25][26] Herein, with an ultrahigh-Ni layered oxide (LiNi 0.94 Co 0.06 O 2 ) as a model cathode and lithium bis(oxalate) borate (LiBOB) as a model electrolyte additive, the EEI chemical and physical property changes on both the cathode and anode are elucidated. Moreover, the layered architecture of the CEI and AEI at the nanometer scale is revealed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) and the correlation between CEI and AEI properties is illustrated. On a chemical perspective, the tuned EEIs are configured with B x O y species and less ethylene carbonate/LiPF 6 decomposition products, which endows the CEI with extreme robustness against As a high-energy-density cathode for Li-ion batteries, high-Ni layered oxides, especially with ultrahigh Ni-content, suffer from short lifespans, due in part to their unstable electrode-electrolyte interphase (EEI). Herein, the cycle life of LiNi 0.94 Co 0.06 O 2 is greatly extended by manipulating the EEI with a lithium bis(oxalate) (LiBOB) additive even when operated at a moderately high voltage (4.4 V vs Li/Li + ). Impressively, the capacity retention can be increased from 61 to 80% after 500 cycles in a full cell paired with a graphite anode. Additionally, the...

A Comprehensive Analysis of the Interphasial and Structural Evolution over Long‐Term Cycling of Ultrahigh‐Nickel Cathodes in Lithium‐Ion Batteries

“…LiNi 0.94 Co 0.06 O 2 is more surface "reactive" and vulnerable under acidic attack, leading to a higher concentration of the electrolyte-decomposition related elements (O, F, P, Ni, and Li) as well as a thicker CEI. [10] Similar trend in the elemental content variation is also revealed in LiNi 0.94 Co 0.06 O 2 after 1500 cycles. [7] Upon successive cycling, for the LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode, the C concentration declines after 500 cycles (Figure 5a), which is mainly due to the reduced content of CC bonding as well as CO bonding as reflected in Figure 5c,d.…”

“…[1] Here, the evolution process of the AEI architecture on the graphite anode paired with different cathodes is revealed. [10] After 100 cycles, F 2 − , O 2 − , and 7 Li − fragments are all located on the top surface of AEI for the graphite paired with LiNi 0.8 Co 0.1 Mn 0.1 O 2 , forming a mono-layer architecture, which is further evidenced by the depth profile in Figure S8 (Supporting Information). [10] After 100 cycles, F 2 − , O 2 − , and 7 Li − fragments are all located on the top surface of AEI for the graphite paired with LiNi 0.8 Co 0.1 Mn 0.1 O 2 , forming a mono-layer architecture, which is further evidenced by the depth profile in Figure S8 (Supporting Information).…”

“…These two fragments mainly result from the acidic attack on the cathode active material or the carbon black. [10,52] The intensity of C 2 O − fragment continually increases at ROI-1 from 100 to 1500 cycles for both cathodes, mainly due to the continuous electrolyte decomposition on the newly exposed surface from active material crack. This majorly arises from the dynamic evolution process of CEI accompanied by mass transfer between carbon black and active material during cycling.…”

“…Figure 4a,b displays the chemical mapping of several fragments of interest on the cross-sectional particles retrieved from fresh and charged cathodes. [10] Furthermore, fluorinated transition-metal species (represented by NiF 2 − ) and carbonate organic decomposition species (represented by C 2 HO − ) are also detected and both are enriched on the particle surface. Interestingly, 6 Li 2 F + fragment (arising from 6 Li/Fcontaining species such as 6 LiF) is mainly distributed on the particle surface, which results from a reaction between residual Li (e.g., 6 LiOH, 6 Li 2 CO 3 , and 6 Li 2 O on the cathode particle surface) and electrolyte (e.g., HF generated from 7 LiPF 6 hydrolysis).…”

“…Initially, a "healthy" graphite AEI is featured with a thin mono-layer architecture. [7,10] Figure 7a,b displays the TOF-SIMS depth profile of C 5 − on the graphite anodes paired with different cathodes. It is also illustrated that the graphite AEI paired with LiNi 0.94 Co 0.06 O 2 suffers from more aggravated degradation with respect to LiNi 0.8 Co 0.1 Mn 0.1 O 2 .…”