Job-Sharing Charge Storage in a Mixed Ion/Electron Conductor Electrode towards 1 Ultrafast Na Storage 2


 For the cathode materials potentially available for high power capability, reducing their particle size can improve the bulk ionic conductivity due to reduced ion diffusion length, and exploiting new reaction mechanism must be fundamentally advantageous. However, other issues such as synthesis difficulty, poor charge storage stability, and capacity decay can emerge. To simultaneously address these issues, in this work, we first find solid-solid interfacial storage for the ultrafine insertion cathode materials in the space-charge region of a mixed ion/electron conductor through the so-called “job-sharing” mechanism. This mechanism shows that electrons and ions can be stored in the different phases around the interface and transport only inside there, which looks thermodynamically distinct from most of conventional charge storage mechanisms in terms of the relationship between charge storage and cell voltage. The insertion cathodes governed by the “job-sharing” mechanism thus exhibit the outstanding performances with high capacity, fast kinetics, and stable cyclability. Herein, the inverse conceptual compositing between ionic conductor and electronic conductor to harness the size effect offers a potential research direction for not only electrode design in high-power batteries, but also other electrochemical potential applications such as solid-state electrolytes and so on.

mechanism shows that electrons and ions can be stored in the different phases around the interface and 22 transport only inside there, which looks thermodynamically distinct from most of conventional charge 23 storage mechanisms in terms of the relationship between charge storage and cell voltage. The insertion 24 cathodes governed by the "job-sharing" mechanism thus exhibit the outstanding performances with high 25 capacity, fast kinetics, and stable cyclability. Herein, the inverse conceptual compositing between ionic 26 conductor and electronic conductor to harness the size effect offers a potential research direction for not 27 only electrode design in high-power batteries, but also other electrochemical potential applications such 28 as solid-state electrolytes and so on.

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Reversible electrochemical systems integrating high energy secondary battery and high power capacitor 31 look ideal, as they can simultaneously meet various demands of mobile electronic devices toward the use 32 for a longer time as well as electric vehicles toward a wider driving range and higher power capability. 33 The demand must be more challenging for sodium ion batteries (SIBs) due to the bigger size of Na + ion 34 (102 pm) than that of Li + ion (76 pm). A simple and straightforward approach to render secondary batteries 35 to be governed by faster kinetics for higher power capability is to reduce the particle size of electrode 36 materials to minimize the distance for ion transfer. 1 Particle miniaturization offers a high surface-to-37 volume ratio, thereby exposing more surface sites as opposed to bulk ones. The surface atoms generally 38 have higher Gibbs free energy and more plentiful defects than its bulk homologue. Hence, the size 39 confinement effect in electrode materials is not limited to shortened ion transfer pathways but can also 40 induce unexpectedly rapid movement of electrons and ions in the enlarged space charge region which 41 may stem from higher contents of surface atoms. 2 Many unique physicochemical characteristics are 42 manifested in size-confined electrode materials, such as the variation of lattice parameter(s), 3-5 chemical 43 potential, 6,7 and electronic structure, 8 all of which can affect their electrochemical behaviors. As for the 44 cathode materials, downsizing is reported to narrow the miscibility gap for phase transitions, facilitating 45 solid solution reactions for improved cyclic stability and reaction kinetics. 9 Moreover, the solid solution 46 reaction facilitated by the size confinement can be typically found in some cathode materials like LiFePO4 47 and LiMn2O4 because of the enhanced Li solubility in their nanocrystalline homologues. 10,11 However, 48 the downsizing strategy is rarely reported for cathode materials, and in many cases, the size confinement 49 with cathode materials often results in the significant decrease of their capacities due to the breakage of 50 internal insertion sites in their crystal structures and the following reduction of inserted ions. 3 Hence, 51 accentuating the merits of size confinement with cathode materials and mitigating its demerits are worth 52 being explored for innovating the cathode materials for alkali ion secondary batteries.

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Novel mechanism for cathode materials should be developed and exploited to innovate the state-of-the-54 art battery technologies. Adjusting the crystal structures of cathode materials can modulate the reaction 55 mechanism toward alkali ions, resultantly improving their energy densities, reversibility, and kinetics. 12-56 15 The introduction of reversible Mn(Ⅱ)/Mn(Ⅳ) redox couple can greatly increase the capacity of lithium-57 excess Mn-based cathode. 13 Putting hetero-species like crystal water and so on into a layered structure 58 (birnessite) cathode exploited the unprecedentedly reversible phase transition between the layered and a 59 metastable spinel-like structure for higher capacity as well as better cyclic stability. 14 However, the 60 number of cathode materials governed by these new mechanisms is few, and so the brand-new mechanism 61 which can be generally applied for a lot of cathode materials is urgently required. Recently, a new 62 interfacial storage mechanism named job-sharing storage which differs from conventional reaction 63 mechanisms like intercalation, conversion, etc. has been proposed by Maier et al. 7,[16][17][18][19] This mechanism 64 reveals that positive and negative charges can be stored separately in two different phases within 65 nanocomposites, contributing additional capacity independent of bulk storage. The reaction kinetics could 66 be ultrafast because the transfer of two charge carriers is also independent. The job-sharing storage was 67 reported to primarily govern the reaction kinetics of transition metal oxides which depend on conversion 68 reaction. However, this mechanism has never been demonstrated in the electrode materials with other 69 reaction mechanisms such as intercalation and so on. Because the job-sharing storage is limited in the 70 space-charge region and thus the following capacity and kinetics are determined by the ratio of boundary 71 region, its universal extension to intercalation and so forth could be accelerated by our size confinement 72 strategy with cathode materials.

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Herein, for the first time, we report the application of this brand-new job-sharing mechanism for size-74 confined insertion-type cathode materials to achieve a high energy, high power, and highly stable battery. 75 We selected Prussian blue analogues (PBAs), one of the commonly used cathodes for SIBs, as a proof of 76 concept to fabricate an artificial mixed conductor by compositing iron hexacyanoferrate nanodots (FeHCF 77 NDs) as an ionic conductor with reduced graphene oxide (rGO) as an electron conductor. This composite 78 was synthesized by a two-step ion-exchange method with FeOOH NDs/rGO composite as a precursor 79 (Fig. 1a, Supplementary Fig. 1-3). The strong interaction between FeHCF NDs and rGO enables the ion 80 and electron pathway to be decoupled within the space-charge region of this composite. The job-sharing 81 mechanism was well clarified, which is totally different from the traditional charge storage mechanisms 82 such as intercalation, conversion and so on. The capacity-voltage relation of interfacial storage in the three 83 modes is updated against that of the bulk one. The FeHCF NDs/rGO composite thus shows beneficial 84 interface/surface behavior and excellent electrochemical performance. Therefore, the mixed electron/ion 85 conductor, FeHCF NDs/rGO, paves a novel way for the charge storage for alkali ion secondary batteries.  Table   115 2), which is partially caused by the increase of bond distance resulting from lattice expansion and may 116 partly be due to the phonon confinement effect within the small sized grains. 3,27 A peak shift to lower 117 binding energy is observed in the Fe 2p X-ray photoelectron spectroscopy (XPS) spectra (  complies with a job-sharing mechanism illustrated in Fig. 3f, which suggests that the ions (e.g., Na + ) can 141 store and transport in one phase (e.g., PB) and electrons in the other phase (e.g., graphene) within the two-142 phase interface. This storage is heavily dependent on size effect and two-phase contacts. The relationship 143 between charge Q stored in the space-charge zone and the particle thickness l (Supplementary Fig. 9) 144 illustrates how Q increases in nanoscale materials and even more so for atomic-scale size. 7 In our case,  Fig. 11). The job-sharing storage contains three modes as given by (for more 157 details, see Methods): (ⅰ) intrinsic characteristics (aNa = const.); (ⅱ) diffusive-layer (aNa ∝ Q n ); and (ⅲ) 158 rigid-layer (aNa ∝ exp(kQ)). To reveal the correlation, we fitted lnQ vs. E plots (Fig. 3g) derived from 159 the discharge curve of FeHCF NDs/rGO at different voltages ( Supplementary Fig. 12). The power-law this regime, the storage shows a capacitor-like behavior that functions in the solid/solid interface.

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One advantage of the job-sharing effect is to provide extra capacity. The insertion capacity of ion 168 insertion type framework is decreasing with the reduction of particle size. The insertion capacity of 169 FeHCF loses nearly 20 mAh g -1 from bulk to a 10nm×10nm×10nm cube (Methods, Supplementary Fig.   170 13, Fig. 3i). However, the two samples show the same capacity at 0.2 C, which is credited to the high 171 interface storage as mentioned above.

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Electrochemical reaction kinetics 173 Mass transfer in the mixed ion/electron conductor could be very fast due to the size confinement effect 174 and the job-sharing mechanism. This is confirmed by the superior rate ability (Fig. 4a)  polarization at high C-rates. Furthermore, FeHCF MCs/rGO shows higher overpotential at high rates, 178 leading to a higher voltage hysteresis ( Supplementary Fig. 14), which are indicative of reduced kinetics 179 and energy efficiency. 33 For comparison, the rate capability of FeHCF NDs/rGO (Fig. 4d) Supplementary Fig. 15a, b, Fig. 4e)  kinetics. In our case, the L for FeHCF MCs is almost 30 times larger than that of FeHCF NDs on average, 198 meaning that the diffusion time of the former is almost 900 times higher than that of the latter, assuming 199 an identical D δ in both cases. On the other hand, diffusion time τ δ also refers to transport resistance R δ and 200 chemical capacitance C δ by the expression τ δ = R δ C δ . 18 Hence D δ is influenced by the two terms, 1/R δ and 201 1/C δ , when L is fixed. D δ is expressed differently for bulks and interfaces (Supplementary Table 3). By 202 inspecting these expressions, two aspects are evident that can rationalize why Dinterface is greater than 203 Dbulk. 18,19 (ⅰ) In bulk materials, the ions and electrons are transferred along the same pathway, whereas in 204 the job-sharing interface, they are decoupled and are transported within their respective conductor (Fig.   205 3f), leading to a low resistance R δ . (ⅱ) For the job-sharing mode, there is an extra term ( 2 2 0 ) related to 206 the electrostatic energy in 1/C δ that reduces the capacitance term C δ .

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The Na ion diffusion coefficient DNa (cm 2 s -1 ) can be calculated via the galvanostatic intermittent 208 titration technique (GITT) (Methods, Supplementary Fig. 16, Fig. 4g). 44,45 FeHCF NDs/rGO overall 209 shows a higher DNa value (10 -8 to 10 -11 cm 2 s -1 ) compared with FeHCF MCs/rGO (10 -10 to 10 -13 cm 2 s -1 ) 210 over both charging and discharging, and is higher than those of many other PBA-based cathodes as FeHCF MCs/rGO, however, the longer diffusion path and the low diffusion coefficient for Na ions induce 215 a diffusion-controlled process at low C-rate. At high current density, which requires short reaction time,

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Na ions probably only diffuse over a short-distance and occupy near-surface reaction sites within FeHCF

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In summary, we implement the first case of job-sharing storage for the insertion-type cathode materials