Electrospun Sn-doped LiTi₂(PO₄)₃/C nanofibers for ultra-fast charging and discharging

Abstract

1 Sn-doped LiTi 2 (PO 4 ) 3 /C composite nanofibers are synthesized by a facile 2 electrospinning process. The unique one dimensional nanostructure combined with 3 uniform electrically conductive carbon matrix allows high-rate transportation of 4 lithium ions and electrons. Besides, Sn-doping could further decrease electrochemical 5 resistance. Sn-doped LiTi 2 (PO 4 ) 3 /C composite nanofibers exhibit excellent 6 electrochemical performance, especially the ultra-fast charging/discharge capability. 7 At a charging rate of about 600 C (64 A g -1 , 6 s), 66.2 % capacity (68.9 mAh g -1 ) 8 could be obtained when matched with Li metal counter electrode. It also presents 9 excellent electrochemical properties as an anode material for aqueous rechargeable 10 lithium batteries. Sn-doped LiTi 2 (PO 4 ) 3 /C composite nanofibers are promising 11 electrode materials for both nonaqueous and aqueous lithium ion batteries. 12 13 14 15 16 17 18 19 20 21 22 23

Research Progress of LiTi2(PO4)3 Anode for Aqueous Lithium-ion Batteries

“…颗粒纳米化是提升电极材料电化学性能的重要 思路 [55][56][57][58][59] 。当材料粒径达到纳米尺度时, 能够有效缩 短 Li + 的扩散路径, 同时抑制缺陷的阻碍作用, 控制 极化现象, 另一方面, 纳米尺度的材料具有较大的 比表面积, 可以增大电极/电解液界面, 进而增加反 应活性位点, 因此颗粒纳米化可以提高材料的倍率 性能和可逆容量 [60] 。纳米尺度的 LiTi 2 (PO 4 ) 3 颗粒主 要通过溶胶-凝胶法、水热法等液相方法合成。 颗粒形貌控制是提高电极材料电性能的另一重 要思路。为获得具有特定形貌的颗粒往往需要采用 水/溶剂热法、静电纺丝法和模板法。Liang 等 [44] 采用 水热法, 在 220 ℃下保温 48 h 合成了具有规则立方 形貌的 LiTi 2 (PO 4 ) 3 , 随着时间延长和温度升高, 颗粒 的微观形貌逐渐呈现规则的方块形状, 最大颗粒接 近 50 μm。Deng 等 [61] 采用溶剂热法合成了具有规则 立方形貌的 LiTi 2 (PO 4 ) 3 , 其粒径尺寸约为 200 nm。 在 10C 倍率下, 其全电池体系 LiTi 2 (PO 4 ) 3 |1 mol•L -1 LiTFSI|LiMn 2 O 4 经 1000 次循环, 容量保持率约为 80%。Liu 等 [24] 将原料置于静电纺丝仪中, 获得了纤 维状前驱体, 经煅烧后获得 LiTi 2 (PO 4 ) 3 纤维材料, 其流程如图 4(a)所示。 与相应的颗粒材料相比, 纤维材料具有比表面 积大和长径比高等优点, 能够有效缩短 Li + 的迁移 距离, 增大材料和电解液的接触面积, 有利于电化 学反应, 展现出更优秀的倍率性能, 由图 4(b)可知, 纤维材料的倍率性能明显优于相应的颗粒材料。…”

“…根据前驱体的种类, 含碳试剂可分为有机碳源 和无机碳源。在高温裂解时, 有机碳源可形成同质化 碳层, 其碳层厚度易于控制, 但石墨化程度通常难 以控制, 无机碳源有助于形成 3D 导电网络结构, 但 对单一颗粒的表面包覆可能效果不佳, 因此同时引 入有机和无机碳源可以提供互补优势 [68] 。有机碳源 主要包括柠檬酸、葡萄糖、蔗糖、淀粉和聚乙烯醇 等, 无机碳源主要有乙炔黑、石墨烯和碳纳米管 (Carbon nanotubes, CNTs)等。 图 4 静电纺丝法示意图(a), LiTi 2 (PO 4 ) 3 纤维与颗粒的倍率性能曲线(b) [24] Fig. 4 Schematic diagram of electrospinning (a), comparison of rate performance between LiTi 2 (PO 4 ) 3 fibers and particles (b) [24] 第 5 期 王禹桐, 等: 水系锂离子电池负极材料 LiTi 2 (PO 4 ) 3 的研究进展 487…”

“…晶格中掺杂 Sn 元素, 但由于合成方法和原料不同, 最佳掺杂量和产物电化学性能也有所差别。Bounar 等 [87] 主要探究了 Sn 元素对晶体结构的影响, 认为在 晶格中, Ti 与 Sn 的最佳摩尔比为 7 : 3, 相应的离子电 导率为 2.17×10 -4 S•cm -1 , 较掺杂前提升了两个数量 级, 适量的掺杂仅引起晶胞参数的线性变化, 而空 间群的种类并未发生改变。 Liu 等 [24] 的合成产物兼具优良的倍率性能和循 环寿命。 他们采用静电纺丝法制备了 LiTi [24] Fig. 6 Discharge capacity for successive cycling at different current densities (a), long-term cycling behavior at current densities of 4 and 6 A•g -1 (b) of LiTi 2 (PO 4 ) 3 /C and LiTi 1.8 Sn 0.2 (PO 4 ) 3 /C [24] 4 结束语…”

The development in aqueous lithium-ion batteries

“…More recently, graphene [62] and carbon nanotube [63] were successively incorporated to LiTi 2 (PO 4 ) 3 , and excellent cycling stability and high rate performance were also presented. Liu et al [64] synthesized Sn-doped LiTi 2 (PO 4 ) 3 /C composite nanofibers via electrospinning method, which delivered the capacity of 101.7 mA g −1 at 4 A g −1 with capacity retention of 99.5 mA g −1 after 10 0 0 cycles, and its capacity retention can reach 66.2% even at high rate of about 600 C (6 s). Wang and co-workers [65] reported that Na doping can increase lattice volume and enhance the diffusion of lithium ions.…”

Flexible All-Solid-State Li-Ion Battery Manufacturable in Ambient Atmosphere

“…NASICON-type anodic LiTi 2 (PO 4 ) 3 and cathodic Li 3 V 2 (PO 4 ) 3 are made of environmentally friendly materials. In addition, they are constructed by stable lattice structures consisting of three-dimensional pathways for Li + transportation, which enable negligible (de)­intercalation strain and fast transportation of Li + while cycling, leading to good rate performance and extended cycle life. , The intrinsically poor electronic conductivity of NASICON-type materials can be mitigated by elemental doping, optimization of particle size, or surface modification in terms of carbon coating. − Regarding the flexibility of electrodes, instead of using a large amount of nonconductive binders, the recent research trend is to fabricate electrodes that possess high specific energy density and flexibility in novel geometric designs without the application of carbon additives, binder, or even conventional metallic current collectors. − Such an electrode is usually made of active material combined with graphene or carbon nanotubes (CNTs) and feature a high in-plane electrical conductivity, an outstanding tensile modulus, and great mechanical endurance. − The elastic feature of carbon-based fabrics can also eliminate the influence caused by volume variations of embedded active materials during charge–discharge and thus preserve the microstructure of electrodes from deformation caused by internal strain . Comparing the application of CNT fabric with graphene in flexible electrodes, advantages of low cost, high flexibility, being substrate-free, and sufficient content of graphitized carbon make CNT fabric highly competitive for large-scale production.…”