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Kim et al. designed a unique organic electrode encapsulating the nitroxide radical polymer, poly(2,2,6,6‐tet(rca)methylpiperidinyloxy‐4‐vinylmethacrylate) (PTMA, 38) into carbon nanotubes (CNT) (d) to form an electrode with high polymer content [71]. To investigate the PTMA‐impregnated CNT electrode, the PTMA–CNT composite electrode was compared as a control sample. The non‐conducting PTMA layer on the PTMA–CNT composite electrode (Figure 9a) blocked the free passage of ions at the Materials 2018, 11, 2567 11 of 18 electrode/electrolyte interface, leading to a larger charge‐transfer resistance of 3716 Ω (Figure 9c). However, the direct contact between CNTs of the PTMA‐impregnated CNT electrode (Figure 9b) improved the electron transfer leading to a decreased resistance of 283 Ω. In addition, the low slope of 2.3.2. Non-Conjugated Conductive Radical Polymers the relationship between the imaginary resistance and the square root of frequency in the low‐ The most widely-explored organic radical polymers for batteries are nitroxide radical polymers frequency region suggested good ion kinetics in the PTMA‐impregnated CNT electrode. The self‐ (Figure 8). The nitroxide radical polymers can be not only reversibly n-doped to aminoxy anions in discharge caused by the dissolution of organic active material can be also minimized effectively by cathodic reactions at relatively high voltage but also p-doped to oxoammonium cations in anodic trapping PTMA in CNTs (Figure 9e), leading to improved discharge capacity, cycling performance and reactions at relative low voltage (Scheme 5). −1 rate Fciagpuarbei7li.t(ya.,b) XRD patterns and (c,d) charge and discharge profiles at a current density of 20 mAꞏg of crystalline oligopyrene (a,c) and amorphous oligopyrene (b,d) as candidate cathode materials for sodium‐ion batteries; Inset of (c) shows the 2nd charge and discharge profile to indicate a large overpotential; Inset of (d) shows the differential capacity (dQ/dV) curves for the 2nd and 10th cycles. Adapted from [68], with permission from © 2013 Elsevier. 2.3.2. Non‐Conjugated Conductive Radical Polymers The most widely‐explored organic radical polymers for batteries are nitroxide radical polymers (Figure 8). The nitroxide radical polymers can be not only reversibly n‐doped to aminoxy anions in cathodic reactions at relatively high voltage but also p‐doped to oxoammonium cations in anodic reactions at relative low voltage (Scheme 5). Figure 8. Molecular structures of two non-conjugated conductive radical polymers. Figure 8. Molecular structures of two non‐conjugated conductive radical polymers. The advantages of the radical polymers include fast kinetics due to the small structural change during the oxidation process, stable cell voltage due to the stable structure, and good processability [72]. However, spontaneous self‐discharging is one of the biggest challenges for radical polymers due Scheme 5. N- and p-doping mechanism of nitroxide radical polymers. to the dissolution iSnctoheemlec5tr. oNly‐ taen,dwph‐dicohpifnugnmcteicohnasnaissmaorfenditorxoxsihdue trtaldeic[7a3l p].oTlyomoevrse.rcome this problem, strategies for reducing the dissolution of radical polymers in electrolytes must be developed. Nakahara et al. first reported a stable nitroxyl polyradical for rechargeable lithium ion batteries Nakahara et al. first reported a stable nitroxyl polyradical for rechargeable lithium ion batteries in It should be noted that the charge‐storage mechanism of the radical polymers involves reversible in 2002, and opened a new field of use for plastic batteries [69]. The first application in NIBs reported 2002, and opened a new field of use for plastic batteries [69]. The first application in NIBs reported in one‐electron redox reaction per repeating unit, and therefore the theoretical capacity of these polymers in 2010 by Dai et al. was a polynorbornene derivative radical polymer, poly[norbornene-2,3-endo, 2010 by Dai et al. was a polynorbornene derivative radical polymer, poly[norbornene‐2,3‐endo, exo‐ is strictly limited by the one‐electron reaction and the molar mass of the repeat unit. To obtain high‐ exo-(COO-4-TEMPO)2] (37) as a cathode active material [70]. The radical polymer 37 delivered an (COO‐4‐TEMPO)2] (37) as a cathode active material [70]. The radical polymer 37 delivered an initial capacity radical polymer electrode materials, it is essential to carefully design the molecular structure initial discharge capacity of 75 mAh·g−1 at a current density of 50 mA·g−1 and retained a discharge discharge capacity of 75 mAhꞏg−1 at a current density of 50 mAꞏg−1 and retained a discharge capacity of of the polymer chain. capacity of 64.5 % at the 50th cycle. 64.5 % at the 50th cycle. Kim et al. designed a unique organic electrode encapsulating the nitroxide radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy-4-vinylmethacrylate) (PTMA, 38) into carbon nanotubes (CNT) to form an electrode with high polymer content [71]. To investigate the PTMA-impregnated CNT electrode, the PTMA–CNT composite electrode was compared as a control sample. The non-conducting PTMA layer on the PTMA–CNT composite electrode (Figure 9a) blocked the free passage of ions at the electrode/electrolyte interface, leading to a larger charge-transfer resistance of 3716 Ω (Figure 9c). However, the direct contact between CNTs of the PTMA-impregnated CNT electrode (Figure 9b) improved the electron transfer leading to a decreased resistance of 283 Ω. In addition, the low slope of the relationship between the imaginary resistance and the square root of frequency in the low-frequency region suggested good ion kinetics in the PTMA-impregnated CNT electrode. The self-discharge caused by the dissolution of organic active material can be also minimized effectively by trapping PTMA in CNTs (Figure 9e), leading to improved discharge capacity, cycling performance and rate capability. The advantages of the radical polymers include fast kinetics due to the small structural change during the oxidation process, stable cell voltage due to the stable structure, and good processability [72]. However, spontaneous self-discharging is one of the biggest challenges for radical polymers due to the dissolution into electrolyte, which functions as a redox shuttle [73]. To overcome this problem, strategies for reducing the dissolution of radical polymers in electrolytes must be developed. It should be noted that the charge-storage mechanism of the radical polymers involves reversible one-electron redox reaction per repeating unit, and therefore the theoretical capacity of these polymers is strictly limited by the one-electron reaction and the molar mass of the repeat unit. To obtain high-capacity radical polymer electrode materials, it is essential to carefully design the molecular structure of the polymer chain.PDF Image | Polymer Electrode Materials for Sodium-ion Batteries
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