Sodium ion secondary battery (SIB) is a low-cost and ubiquitous secondary battery for next-generation large-scale energy storage. The diffusion process of large Na+ (ionic radius is 1.12 Å), however, is considered to be slower than that of small Li+ (0.76 Å). This would be a serious disadvantage of SIB as compared with the Lithium ion secondary battery (LIB). By means of the electrochemical impedance spectroscopy (EIS), we determined the diffusion constant (D) of Na+ in thin films of O3- and P2-type NaCoO2 with layered structures. We found that the D values (~ 0.5–1.5 × 10−10 cm2/s) of Na+ are higher than those (< 1 × 10−11 cm2/s) of Li+ in layered LiCoO2. Especially, the D values of O3-NaCoO2 are even higher than those of P2-NaCoO2, probably because O3-NaCoO2 shows successive structural phase transitions from the O3, O’3, P’3, to P3 phases with Na+ deintercalation. We further found that the activation energy (ED ~ 0.4 eV) for the Na+ diffusion is significantly low in these layered cobalt oxides. We found a close relation between the relative capacity and the renormalized discharge rate ( = L2/DT, where L and T are the film thickness and discharge time, respectively).

We demonstrate that a sodium-ion secondary battery (SIB)-type thermocell consisting of two types of Prussian blue analogue (PBA) with different electrochemical thermoelectric coefficients (SEC ≡ ∂V/∂T; V and T are the redox potential and temperature, respectively) produces electrical energy during heat cycles. The device produces an electrical energy of 2.3 meV/PBA per heat cycle between 295 K (= TL) and 323 K (= TH). The ideal thermal efficiency (η = 1.0%), which is evaluated using the heat capacity (C = 4.16 meV/K) of ideal Na2Co[Fe(CN)6], reaches 11% of the Carnot efficiency (ηth = 8.7%). Our SIB-type thermocell is a promising thermoelectric device that harvests waste heat near room temperature.

A thermocell that consists of cathode and anode materials with different temperature coefficients (α = dV/dT) of the redox potential (V) can convert environmental thermal energy to electric energy via the so-called thermal charging effect. The output voltage Vcell of the current thermocell, however, is still low (several tens mV) and depends on temperature, which are serious drawbacks for practical use of the device as an independent power supply. Here, we report that usage of phase transition material as electrode qualitatively improve the device performance. We set the critical temperature (Tc) for the phase transition in cobalt Prussian blue analogue (Co-PBA; NaxCo[Fe(CN)6]y) to just above room temperature, by finely adjusting the Fe concentration (y = 0.82). With increase in the cell temperature (Tcell), Vcell of the NaxCo[Fe(CN)6]0.82 (NCF82)/NaxCo[Fe(CN)6]0.9 (NCF90) cell steeply increases from 0 mV to ~120 mV around 320 K. Our observation indicates that the thermocell with use of phase transition is a promising energy harvesting device.

Tertiary battery is charged by the environmental heat, not by the electric energy, by using the difference (Δα) in the thermal coefficient (α=dV/dT) of redox potential (V) between the anode and cathode materials. The thermal cyclability is not good in the prototypical NaxCo[Fe(CN)6]0.71 (NCF71)/NaxCo[Fe(CN)6]0.90 (NCF90) tertiary battery. Here, we significantly improved the thermal cyclability of the tertiary battery with using Ni‐substituted NaxNi[Fe(CN)6]0.68 (NNF68). The Ni‐substituted NNF68/NCF90 tertiary battery shows good thermal cyclability: both the cell voltage and capacity essentially unchanged up to the 10th thermal cycles.

application range of the battery materials from energy storage to energy conversion. The exploration of high-α material with different sign is quite effective to fabricate a high-η thermocell.Among the battery materials, PBAs, whose chemical formula are Li x M[Fe(CN) 6 ] y and Na x M[Fe(CN) 6 ] y (M is transition metal), are promising candidates for the cathode materials in lithium-ion/sodium-ion secondary batteries. [8] For example, thin film of Li 1.6 Co[Fe(CN) 6 ] 0.9 shows a high capacity of 132 mAh/g with a good cyclability. Most of PBAs have the face-centered cubic (fcc) (Fm3m; Z = 4) or trigonal (R3m; Z = 3) structures. [9] They consist of three-dimensional (3D) junglegym-type host framework and guest Li + /Na + ions and H 2 O molecules, which are accommodated in nanopores of the framework. The framework contains considerable [Fe(CN) 6 ] vacancies (10-30 %). The discharge curves of CoÀ and Mn-PBA show characteristic plateaus, whose redox reactions are well assigned by means of X-ray absorption spectroscopy. [10,11] Recently, the α values are reported in Li x Co[Fe(CN) 6 ] 0.71 and Figure 2. Discharge curves of (a) Na x Co[Fe(CN) 6 ] 0.71 (NCF71), (b) Na x Co[Fe (CN) 6 ] 0.9 (NCF90) and (c) Na x Mn[Fe(CN) 6 ] 0.83 (NCF83) films measured at 0.1 C. For convenience of explanation, we defined plateaus I, II, II, IV, and V.