Tuning (003) interplanar space by boric acid co-sintering to enhance Li+ storage and transfer in Li(Ni0.8Co0.1Mn0.1)O2 cathode

Highlights: • Ni-rich cathode materials for lithium ion battery. • Synthesis mechanism establishment for NMC811. • Tuning (003) spacing via boron-ion doping enhancing performance. • Ex-situ XRD for phase transformation during charging–discharging process. -- Abstract: High nickel layered oxides (LiNi_0.8Co_0.1Mn_0.1O_2, NMC811) demonstrate high capacity (≥ 200 mA h/g) and low cost, making them increasingly attractive as a high energy active cathode to replace LiCoO₂ or LiFePO₄ for lithium ion batteries. Nevertheless, their large scale practical applications are still inhibited by the low structural stability, insufficient rate capability and cyclability. In this work, a stable and safe LiNi0.8Co0.1Mn0.1O2 cathode was synthesized by facile oxalic acid assisted co-precipitation method. Boric acid has been chosen and utilized as a co-sintering agent to enhance the electrochemical performance via fine tuning internal interplanar space between transition metal oxide (003) layers. The B03 sample with 3.0 at% of boron doping exhibited a sustainable reversible capacity as high as 220 mA h g⁻¹ with approximate 97% capacity retention after 100 cycles at 0.5 C charging/discharging rate. It also delivered 93 mA h/g under 10 C rate testing. In the long-term cycling test, this B03-NMC 811 cathode demonstrated 83% capacity retention after 300 cycles at 5 C cycling test. Appropriate adding of boron not only enhanced their C-rate capability and cyclability but also improved their structural stability during repetitive charging and discharging process through ex situ XRD measurement. The c axis variation of B03-NMC 811 cathode between 3.7 V and 4.3 V was reduced by 7.6% (0.014 Å) as compared to the pristine one. However, larger quantity (5.0 at%) of boron degrades the electrochemical performance induced by increased Li/Ni disorder. Thereafter, this enhancement in both electrochemical performance and structural stability benefits from the optimized interplanar space between transition metal oxide (003) layers, which can be tuned precisely by such a simple but novel co-sintering process with boric acid. This approach opens the door to structural modification and optimization of NMC811 cathode to satisfy the demanding performance and safety requirements of electric vehicles.