1. Introduction
The development of portable electronics and electric vehicles has
stimulated a huge demand for rechargeable lithium batteries because of
their high energy density and high safety
requirements.[1] To continuously improve the
energy density of lithium batteries, various cathode and anode materials
with high specific capacities have been explored. Among them, Ni-rich
layered transition metal oxide
LiNixCoyMnzO2(NCM, x ≥ 0.8, x + y + z = 1) has become a key cathode material to be
developed for applications with its reasonable cost and high practical
energy density.[2,3] However, several problems
still need to be overcome to meet practical demands, including
insufficient cycling stability and poor rate ability. These problems all
stem from Li+/Ni2+ cation mixing,
poor structural stability, and harmful side reactions at the
electrode/electrolyte interface.[4-6] Surface side
reactions might cause a significant increase in interfacial resistance
as well as severe dissolution of transition metals, especially at high
temperatures and increased anodic cut-off
voltage.[7-9] Therefore, side reactions at the
electrode/electrolyte interface remain the limiting factor for
performance.
Protective particle coatings, mainly including metal
oxides,[10-13] oxide solid
electrolytes,[14-16]phosphates,[6,17,18]fluorides,[19-22] or
graphene,[23-25] were widely utilized to address
these problems. For example, Xu et al.[26] used
Al2O3 and Zhou et
al.[10] utilized ZnO. In recent years,
two-dimensional (2D) materials, such as graphene and boron nitride, were
used as electrode coating materials due to their high specific surface
area, and adjustable physicochemical
properties,[27] which could significantly enhance
the electrochemical performance of cathode materials. However,
ex-situ-added 2D materials tended to restack and aggregate in the
cathode, which seriously slowed down the lithium-ion transport rate and
reduced the active surface of the active
materials.[28,29] Moreover, commonly used 2D
materials often exhibited poor lithium-ion conductivity, which hindered
the effective transfer of lithium ions inside the cathode. Selecting the
2D material with excellent lithium conductivity and achieving its
uniform coating on the surface of cathode particles is crucial for
improving the electrochemical performance of the cathode.
g-C3N4 consists of a continuous 3-s
triazine ring (C6N7) as the basic unit
and is thought to possess a two-dimensional layered structure similar to
graphene.[30] C and N atoms are sp2-hybridised,
forming a highly delocalised p-conjugated
system.[31] The unique nano-pore structure and
abundant nitrogen vacancy edges in
g-C3N4 facilitate the adsorption and
diffusion of lithium ions.[32,33] Moreover, the
mild synthesis conditions make it suitable for uniform in-situ coating
of matrix electrode materials. Therefore, the utilization of 2D
thin-layer g-C3N4 for uniform coating of
NCM cathode particles is expected to suppress side reactions at the
cathode/electrolyte interface without affecting the lithium-ion
transport, thereby improving the electrochemical performance of NCM
cathodes.
Herein, g-C3N4-coated
LiNi0.8Co0.1Mn0.1O2(NCM811@CN) is constructed to alleviate the side reaction and improve
the cycle performance. The g-C3N4coating effectively inhibits structural degradation and intergranular
cracks. Meanwhile, strong chemical bonds form between
g-C3N4 and the NCM cathode due to the
unsaturated bonds on the surface of
g-C3N4,[34,35]which not only improves the interface stability and accelerates the
interfacial Li+ diffusion but also avoids the direct
contact between the cathode and the electrolyte, suppressing the side
reactions. Benefiting from the above advantages, the optimized
NCM811@CN-3wt% cathode exhibits better rate capability and cycling
stability than the uncoated NCM811 in both conventional liquid and
solid-liquid-hybrid electrolyte-based cells. This work sparks new ideas
on constructing ultra-thin interface layers with excellent comprehensive
performance for high-nickel ternary-metal oxide cathodes.