碳材料，特别是石墨化碳质材料，作为钾离子电池（PIBs）的负极，是PIBs产业化的基石。然而，石墨化碳材料PIBs负极通常循环寿命短，稳定性差。
基于以上现状，湖南大学的张二晋等在国际知名期刊Advanced Science上发表题为“Carbon Dots@rGO Paper as Freestanding and Flexible Potassium-Ion Batteries Anode”的论文。张二晋博士为本文第一作者。
在本研究中，通过在还原氧化石墨烯表面引入碳点（CDs@rGO），构建了柔性三维结构的高性能钾离子电池负极。该三维结构的CDs@rGO薄膜具有利于电解液进入活性位点的通道，可有效增强电子/离子的传输和扩散。此外，CDs引入的大量缺陷和含氧官能团，可有效提升比容量，提高倍率性能，改善电化学性能。
这种独立且灵活的CDs@rGO负极在100 mA g-1的电流密度下具有310 mAh g-1的高比容量，初始库伦效率（ICE）约为44.4％；在200 mA g-1的电流密度下循环840次后仍保持244 mAh g-1的比容量；即使经历连续6次电流变化后，在500 mA g-1的高电流密度下也可以提供185 mAh g-1的比容量。通过引入碳点来增强电化学性能的策略可以为其他研究领域提供新的思路和视野。
由于资源储量丰富，能量密度高，钾离子电池（PIB）成为未来替代锂离子电池的能源存储设备的热门之一。MoSe2，MoS2，FeS2，CuS，CoS，VSe，Sb，和Bi等材料已被广泛用于高容量PIBs负极的研究。然而，由于传输动力学差、不可逆合金化效应和巨大的体积膨胀等因素限制了它们的应用。石墨是一种工业化程度高、加工技术成熟、成本低的电极材料，可以通过钾离子的嵌入形成一级插层化合物（GIC，KC8），其理论容量为279 mAh g-1。然而，循环寿命短和倍率性能不理想是阻碍石墨负极大规模应用的关键障碍。
研究人员已通过掺杂，纳米结构设计和电解质工程等策略来增强石墨负极的电化学性能。例如，氮掺杂的石墨烯可以将PIBs的容量从石墨的理论容量增加到超过350 mAh g-1。硬-软碳复合负极在440次循环后容量保持率可以达到89％。碳纳米笼和中空聚吡咯碳负极分别实现了钾离子的快速存储等。
此外，我们也已经证明4 M/L双氟磺酰基亚胺钾（KFSI）的高浓盐电解液可以实现石墨负极循环一年以上（约1700个循环）。然而，高浓盐电解液成本高，且KFSI基电解液会腐蚀铝箔（集流体）,相比之下，基于六氟磷酸钾（KPF6）的传统低浓度碳酸盐电解液通常具有化学稳定性好和成本低等优点。因此，基于传统电解液的高性能钾离子石墨负极还亟待开发。
本这项工作中，作者利用微波辅助将碳点锚定在还原氧化石墨烯表面（CDs@rGO）,系统研究了碳点对还原氧化石墨烯结构、电化学和钾离子储存性能的影响。
研究发现碳点的引入对氧化石墨烯表面结构及化学键有很大影响，并导致电化学性能的变化。电化学性能测试结果结合循环伏安法及原位XRD测试表明，碳点的引入使还原氧化石墨烯的储钾机制从嵌钾/脱钾转变为嵌钾/脱钾与吸附/脱附共同作用，显著提高其比容量，改善循环性能和倍率性能，使还原氧化石墨烯快速可逆储K+成为可能。
a) Prepared flexible, freestanding CDs@rGO film paper.
b and c) Top view SEM images of CDs@rGO film paper (Red mark: fissures).
d and e) Cross-section SEM images of CDs@rGO film paper.
f) TEM and g) HRTEM images of the CDs@rGO paper.
h) The size distribution histogram of CDs anchoraed on rGO, over 50 carbon dots were counted for statistics.
i) Schematic diagram of CDs@rGO stacked models.
j) XRD patterns of CDs@rGO film paper.
k) Raman spectra of CDs@rGO film paper.
l) The XPS spectra of pristine CDs@rGO film.
a) CV curves at the scan rate of 1 mV s−1.
b) Charge-discharge profile at 100 mA g–1.
c) Corresponding dQ/dV profiles of the initial 2 cycles.
d) Rate performance at various current densities from 100 to 500 mA g–1 after activation at 100 mA g–1.
e) Corresponding charge-discharge profiles at 100 to 500 mA g–1.
f) Long-term cycling performance at 200 mAh g–1.
g) Comparison of the specific capacity under different currents between the CDs@rGO film paper and other reported carbonaceous electrodes for PIBs.
a) The CV curves for CDs@rGO half-cells from 0.1 to 0.4 mV/s.
b) b-value determination for the peak cathodic currents.
c) Capacitive (red) and diffusion-controlled (gray) contribution of the CDs@rGO electrode to charge storage at 0.1 mV/s.
d) Normalized contribution ratio of capacitive (red) and diffusion-controlled (gray) capacities at different scan rates.
e) The EIS tests of K/CDs@rGO batteries;
f) The EIS tests of K/rGO batteries.
a-c) Contour plot of the operando XRD results of CDs@rGO film anode during discharging/charging process for the initial three cycles. The XPS spectra of pristine CDs@rGO film anode and anodes from half-cell after 8 cycles:
h) C, K, F, and P elements distribute from full charged CDs@rGO electrode.
i) The binding energy of the K ions with O-related groups.
j) Model structures for the binding energy calculation of a K ion and C-OH groups.
Carbon Dots@rGO Paper as Freestanding and Flexible Potassium-Ion Batteries Anode
https://onlinelibrary.wiley.com/doi/10.1002/advs.202000470
张二晋在博士期间，主要从事碳基电极材料研究，其以第一作者（包括共同一作）发表的碳基电极材料相关论文包括：
[1] Zhang Erjin, Wang Bin, Yu Xinzhi, Zhu Jingyi, Wang Longlu, Lu Bingan*, β-FeOOH on carbon nanotubes as a cathode material for Na-ion batteries. Energy Storage Materials 2017, 8, 147
[2] Zhang Erjin, Cao Wei, Wang Bin, Yu Xinzhi, Wang Longlu, Xu Zhi, Lu Bingan*, A novel aluminum dual-ion battery. Energy Storage Materials 2018, 11, 91
[3] Zhang Erjin, Wang Jue, Wang Bin, Yu Xinzhi, Yang Hongguan, Lu Bingan*, Unzipped carbon nanotubes for aluminum battery. Energy Storage Materials 2019, 23, 72
[4] Zhang Erjin, Wang Bin, Wang Jue, Ding Hongbo, Zhang Shi, Duan Huigao, Yu Xinzhi, Lu Bingan*, Rapidly synthesizing interconnected carbon nanocage by microwave toward high-performance aluminum batteries. Chemical Engineering Journal 2020, 389, 124407
[5] Erjin Zhang, Xinxin Jia, Bin Wang, Jue Wang, Xinzhi Yu, and Bingan Lu*. Carbon Dots@rGO Paper as Freestanding and Flexible Potassium-Ion Batteries Anode. Advanced Science, 2020, Doi/10.1002/advs.202000470
[6] Jia Xinxin#, Zhang Erjin#, Yu Xinzhi*, Lu Bingan, Facile Synthesis of Copper Sulfide Nanosheet@Graphene Oxide for the Anode of Potassium‐Ion Batteries. Energy Technology 2019, 8, 1900987
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