Engineering of interfaces in sodium-ion batteries for long cycle life

Song, Tengfei ORCID: 0000-0002-8512-0081 (2024). Engineering of interfaces in sodium-ion batteries for long cycle life. University of Birmingham. Ph.D.

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Abstract

Sodium-ion Batteries (SIBs) present a wealth of possibilities for cost-effective and environmentally friendly energy storage solutions. One challenge to close the gap is the insufficient cycling lifetime.

To maximize their potential, new cathode materials with high specific capacity and stable structures are required. Cobalt-free sodium transition metal oxides of O3 type are promising candidates as cathodes for use in SIBs, owing to their high specific capacity, appropriate operating potential, wide availability of transition metals, and facile synthesis methods that can be inherited from lithium-ion batteries. However, layered oxides suffer from irreversible phase transitions and structural instability, hindering long-term cycling stability. In this work, innovative approaches for improving the cycle life of layered oxides (O3-type NaNi1/3Fe1/3Mn1/3O2 in specific) and the final full-cell battery are investigated through structure tailoring, morphology engineering, and full-cell design. Through in-depth analysis and experimentation, this research aims to establish an effective path toward the practical utilization of enhanced sodium-ion batteries.

Firstly, the flexibility of Sn4+ dopant, a non-transition metal, was explored, with the aim of modifying the material’s bulk structure by suppressing the host rearrangement and TMO2 gliding. A series of Sn-modified NaNi1/3Fe1/3Mn1/3O2 cathode was synthesized and characterized, and 8%Sn-modified NaNi1/3Fe1/3Mn1/3O2 cathode exhibited outstanding cycling performance owed to the simultaneously stabilized bulk structure and interface. The underlying mechanism was studied through an extensive analysis (Chapter 4).

Secondly, the physical properties of the synthesized materials are significantly dependent on the synthetic conditions, which in turn affect the final electrochemical performance. Here, the effect of synthesis conditions (pH, molar ratio of ammonia/metal precursor salt, and stirring speed) upon the particle size and morphology of NaNi1/3Fe1/3Mn1/3O2 particles was investigated for the first time. Statistical methods and regression analysis were employed to provide valuable insights into the relationship between synthesis parameters and materials properties. Optimized conditions were derived from the obtained regression models, and particles synthesized with optimum parameters displayed uniformly distributed particle size together with a threefold increase in cycling performance (Chapter 5).

Finally, the optimized NaNi1/3Fe1/3Mn1/3O2 cathode synthesized under the optimum parameters together with 8% Sn modification was fabricated into 8%Sn-NaNi1/3Fe1/3Mn1/3O2 || Hard carbon full cells to verify the practical application. The effects of the N/P ratio (areal capacity ratio between negative and positive electrode) on the material utilization, initial capacity, first Coulombic efficiency, and cycling performance were investigated by using three-electrode cells. The failure mechanisms under different N/P levels were preliminary discussed. The fundamental reason for battery failure is that the electrode potential of the positive or negative electrode exceeds the critical window. Cells with an N/P ratio of 0.9 perform the best cycling performance due to the suppression of irreversible phase transition and sluggish kinetic reactions in the cathode at voltages above 4.1 V (Chapter 6).

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):