Critical Problems and Modification Strategies of Realizing High‐Voltage LiCoO₂ Cathode from Electrolyte Engineering

Abstract

As the pursuit of greater energy density for the portable battery has stimulated exhaustive research in high‐voltage lithium‐ion batteries (LIBs), developing electrolyte additives is considered a cost‐efficient way to improve the performance of the battery. Here, the three interactional issues of LiCoO2 (LCO) batteries in commercial electrolytes at high voltage are summarized, this review first identifies an unavoidable vicious cycle of the commercial electrolyte at high voltage. LCO/electrolyte interphase break, dissolution of transition metal (TM) ions, and formation of harmful HF accelerate the failing progress of the commercial electrolyte at high voltage, besides the malfunction of anode happens at the same time because of electrode crosstalk. Then, the modification of the commercial electrolyte is summarized according to the solutions of this vicious cycle. Last, a framework for future research on high‐voltage LCO batteries is outlined.

Weakened Solvation Structure Electrolytes Enable High-Voltage Graphite||LiCoO2 Batteries

“…In pursuit of higher energy and power density, lithium-ion batteries (LIBs) require more advanced electrolytes. − Since 1991, the discovery of the cyclic ethylene carbonate (EC)-based electrolytes has demonstrated substantial compatibility with commercial electrodes. − This is mainly due to the strong interaction between EC and Li + , which results in its greater involvement in the solvation structure, while other linear carbonates are less involved. , Therefore, the EC surrounding Li + will be reduced first to form a stable solid electrolyte interphase (SEI).…”

Modulating Solvent Layer Structure of Li+ to Achieve a Stable High-Voltage Electrolyte for Lithium-Ion Batteries

“…However, as high-voltage charging progresses (>4.2 V vs Li/ Li), 3 commonly used carbonate-based electrolytes containing ethylene carbonate (EC) and diethyl carbonate (DEC) are easy to oxidize, leading to the thickening of the passivation layer, loss of active lithium, and dissolution of transition metals, which can cause electrode structure instability. 13,14 Moreover, the electrodes may experience surface degradation, damage caused by harmful phase transitions, and uneven reactions and are also prone to oxidative decomposition at high voltages, leading to rapid declines in capacity, efficiency, and cycle life.…”

“…Among these, high-voltage cathode materials (>4.5 V vs Li/Li + ), such as lithium-rich compounds, olivine-type LiMPO 4 (M is Ni, Co), , and spinel-type LiCoMnO 4 and LiNi x Co y Mn 1‑x‑y O 2 , are of particular interest. , Among them, LiCoO 2 cathode (LCO) materials are preferred for portable electronic devices due to their higher redox potential difference, higher energy density, − and good electrochemical performance and energy storage characteristics. However, as high-voltage charging progresses (>4.2 V vs Li/Li), commonly used carbonate-based electrolytes containing ethylene carbonate (EC) and diethyl carbonate (DEC) are easy to oxidize, leading to the thickening of the passivation layer, loss of active lithium, and dissolution of transition metals, which can cause electrode structure instability. , Moreover, the electrodes may experience surface degradation, damage caused by harmful phase transitions, and uneven reactions and are also prone to oxidative decomposition at high voltages, leading to rapid declines in capacity, efficiency, and cycle life.…”

H-Transfer Mediated Self-Enhanced Interphase for High-Voltage Lithium-Ion Batteries

“…Although state-of-the-art lithium-ion batteries (LIBs) have gained considerable commercial success and technological advances over recent years, they still face many challenges such as less-than-expected energy density, safety issues, and narrow temperature operation range. − Researchers have explored various methods to meet the growing demand for energy storage devices, among which the utilization of Ni-rich layered oxides, such as the LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811), has become a prevalent approach to achieving higher energy density. − Unfortunately, the adoption of such aggressive cathode materials poses compatibility challenges with traditional low-concentration LiPF 6 /carbonate-solvent based electrolytes. − Specifically, conventional carbonate electrolytes fail to form a stable cathode/electrolyte interphase (CEI) at voltages above 4.3 V, leading to rapid capacity degradation. ,, For instance, ethyl methyl carbonate (EMC) and ethylene carbonate (EC), widely employed solvents in organic electrolytes, tend to form a porous and unstable CEI. Multiple research projects have been committed to explore the mechanisms underlying carbonate electrolyte oxidization, particularly emphasizing the pivotal role of H-transfer reactions. − Due to their small size, H radicals exhibit high transferability among different chemical species, i.e., H transfer reactions. , In carbonate electrolytes, such H-transfer reactions often occur on NMC cathodes with a high charging state, where solvents (such as EC and EMC) undergo dehydrogenation first to produce H radicals, which would further form trace water, resulting in the hydrolysis of PF 6 – anions and acidic HF .…”