Thick electrode design for lithium-ion batteries from an ion-electron coupled transport perspective: from independent regulation to cooperative design

INTRODUCTION

To address the stagnation of energy density improvement in conventional lithium-ion batteries, thick electrode design has emerged as a promising device-level strategy^[1-3]. By increasing electrode thickness, higher gravimetric and volumetric energy densities can be achieved, accompanied by simplified stacking configurations and reduced manufacturing costs^[4-7]. Nevertheless, the transition from thin to thick electrodes fundamentally alters the internal transport environment, introducing severe kinetic penalties. As thickness increases, ion and electron transport pathways become substantially elongated and more tortuous due to the random assembly of multicomponent phases^[8,9]. The resulting heterogeneous pore networks and fragile conductive contacts lead to pronounced concentration polarization, internal resistance, and reaction heterogeneity^[10]. Under such conditions, electrochemical activity is often confined near the separator or current collector. Consequently, transport kinetics, rather than intrinsic material properties, become the primary bottleneck limiting the practical energy and power performance of thick electrodes.

Among the two charge carriers, sluggish ion transport is widely regarded as the dominant limitation in thick electrodes. To mitigate this issue, extensive efforts have been devoted to engineering fast ion transport pathways across multiple length scales^[11,12]. At the particle level, reducing solid-state diffusion lengths through particle downsizing or constructing hierarchical porous architectures has proven effective in enhancing electrolyte accessibility^[13]. At the mesoscale, directional pore engineering strategies enable vertically aligned channels that significantly reduce tortuosity and facilitate rapid ion transport^[14]. At the electrode scale, gradient architectures in porosity, particle size, or composition are employed to spatially match ionic flux with local reaction demand, alleviating ion depletion and homogenizing reaction distributions^[15]. Collectively, these strategies demonstrate that rational structural design can partially compensate for the intrinsic transport limitations imposed by increased thickness.

In parallel, efficient electron transport is equally indispensable, yet it is often more susceptible to mechanical deformation and interfacial degradation during cycling^[16]. Conventional electronic networks formed by zero-dimensional conductive additives rely on fragile point-to-point contacts. To address this challenge, conductive network design has evolved toward higher-dimensional and hierarchical architectures. One-dimensional carbon nanotubes and two-dimensional graphene sheets establish long-range conductive bridges and low-resistance interfacial contacts^[17]. Synergistic combinations of multidimensional conductive agents further ensure electronic connectivity across particle, mesoscale, and electrode-scale domains. For ultrathick electrodes, three-dimensional porous current collectors and integrated conductive scaffolds provide continuous electronic backbones and mechanical reinforcement, transforming inefficient interparticle hopping into direct backbone conduction^[18]. Additional interfacial strategies, including surface conductive coatings and controlled dispersion of conductive additives, further reduce charge-transfer resistance and stabilize electronic pathways^[19].

Despite substantial progress, most existing strategies still focus on independently accelerating either ionic or electronic transport. However, electrode performance is governed by the synchronous supply of ions and electrons to electrochemical reaction interfaces. Mismatch between ionic and electronic fluxes inevitably leads to localized polarization, reaction heterogeneity, and premature capacity loss, even when one transport pathway is highly optimized. This limitation underscores the necessity of moving beyond isolated pathway engineering toward a coupled ion-electron transport perspective.

ION-ELECTRON COOPERATIVE TRANSPORT MECHANISMS AND PRINCIPLES

Under large overpotentials, reaction currents no longer follow the exponential dependence predicted by classical kinetics. Instead, they exhibit saturation behavior, indicating that interfacial reaction kinetics become rate-limiting even when macroscopic transport pathways appear unobstructed^[20]. The conventional Butler-Volmer framework, which treats ionic and electronic transport as independent sequential processes, fails to capture this phenomenon. Bazant et al. pointed out that Butler-Volmer inadequately describes high-current behavior in embedded electrode materials because they neglect ion-electron interactions within the transition state of the reaction^[21]. To resolve this discrepancy, the coupled ion-electron transfer (CIET) theory provides a revised microscopic description of interfacial reactions. CIET emphasizes that Li⁺ intercalation is an intrinsically cooperative process requiring the simultaneous insertion of ions and electrons. Rather than proceeding along a single reaction coordinate, the interfacial reaction follows a minimum-free-energy pathway on a two-dimensional energy landscape defined by ion site availability and electronic reorganization [Figure 1A]. Consequently, reaction kinetics are constrained by the slower of the two processes.

Figure 1. Mechanism of ion-electron coupled transport at different scales. (A) CIET theory at the active material interface on the microscopic scale. This figure is quoted with permission from Ref.^[21], Copyright © 2025 American Association for the Advancement of Science; (B) Regulation of ion and electron transport capabilities via a calendering strategy; (C) Heterogeneous lithiumation distribution in electrodes with varying porosities; (D) Capacity retention of thin and thick electrodes at different porosities; (E) The hybrid ion-electron control mechanism ensuring that the electrode remains within the capacity safety zone. (B-E) are quoted with permission from Ref.^[22], Copyright © 2024 Wiley. CIET: Coupled ion-electron transfer; IT: interfacial transfer; ET: electron transfer; SOC: state of charge.

This microscopic coupling inevitably manifests at the macroscopic scale as a matching requirement between ionic and electronic transport fluxes. Fu et al. demonstrated that unbalanced transport properties lead to spatial response heterogeneity [Figure 1B]^[22]. When porosity is excessively high, ionic resistance is minimized but electronic percolation is weakened, rendering regions near the separator electrochemically inactive. Conversely, overly dense electrodes enhance electronic conduction but impede ion transport [Figure 1C and D]. Uniform reaction distribution is achieved only when ionic and electronic transport capabilities are balanced within the hybrid control zone. Importantly, this balance becomes increasingly sensitive to structural parameters as thickness increases [Figure 1E], indicating that electrode performance is governed not by the independent maximization of either transport pathway, but by their dynamic balance.

To translate ion-electron cooperative transport from qualitative insight into quantitative design, a mechanistic linkage between microstructure and macroscopic kinetics is required. The Damköhler number (D_a), defined as the ratio of characteristic diffusion time to characteristic reaction time, is commonly employed to assess transport limitations in thick electrodes. However, conventional models typically assume electronic transport to be non-limiting and thus fail to capture coupled transport effects. Within the CIET framework, the total transport resistance must instead account for both ionic and electronic contributions. Building on this concept, modified D_a have been developed to describe reaction current distributions as a function of effective ionic and electronic conductivities^[23]. When the depletion time associated with the initial electrolyte concentration (c₀) is used as the characteristic reaction timescale, a coupled D_a can be derived that explicitly incorporates electrode thickness (L), applied current density (I_app), and effective transport coefficients (D_eff) for both charge carriers^[24].

where τ_couple denotes the characteristic diffusion time after considering ion-electron coupling, τ_rxn denotes the characteristic reaction time after considering ion-electron coupling, and F indicates Faraday constant.

To relate this coupled kinetic criterion to electrode microstructure, porous media theory is employed. The effective ionic transport coefficient scales with porosity (ε) and tortuosity (τ). Combining these relations yields a direct structure-performance correlation that quantitatively captures hybrid control zone.

At low porosity, severe pore blockage leads to ion-transport divergence and an ion-limited regime (D_a≫1). At high porosity, disruption of the electronic network results in an electron-limited regime, again characterized by D_a≫1. In contrast, the hybrid control zone is achieved when ionic and electronic transport timescales are dynamically matched, corresponding to D_a ≈ 1. By converting the ion-electron cooperative mechanism into explicit constraints on porosity and tortuosity, this framework establishes a clear theoretical boundary for thick-electrode optimization.

STRATEGIES FOR CONSTRUCTING ION-ELECTRON COOPERATIVE TRANSPORT

Building on microscopic CIET theory and the macroscopic ion-electron hybrid control mechanism, the design of high-performance thick electrodes should prioritize precise matching between ionic and electronic transport networks, rather than isolated enhancement of individual pathways. Table 1 summarizes the respective advantages and disadvantages of the three enhancement strategies: ion transport, electron transport, and coupled transport. The strategies focusing solely on ion transport or electron transport inevitably introduce counteracting limitations: increasing porosity and reducing tortuosity alleviate concentration polarization but often disrupt electronic connectivity, whereas densifying electrodes and increasing conductive additives improve electronic conduction at the cost of ion blockage. In conventional thick electrodes, random packing of multiphase components leads to pore collapse, conductive network degradation, and increased tortuosity during cycling. An ideal thick-electrode architecture should therefore integrate a continuous, low-tortuosity pore network for ion transport with a robust, low-resistance solid conductive skeleton for electron transport. This design ensures that all active sites are simultaneously well-wetted by the electrolyte and efficiently connected to the external circuit [Figure 2A].

Figure 2. High-performance thick electrodes for constructing ion-electron cooperative transport networks. (A) Schematic of ion-electron transport pathways in conventional electrodes versus desired electrodes; (B) Electrostatic self-assembly enabling spatial decoupling of ion/electron transport pathways. (A and B) is quoted with permission from Ref.^[26], Copyright © 2018 Wiley; (C) Two-step synthesis yielding an Nb₂O₅/HGF composite architecture with independent three-dimensional hierarchical pores; (D) SEM image of the three-dimensional hierarchical porous structure; (E) Comparison of capacity retention rates for electrodes with different loading levels. (C-E) are quoted with permission from Ref.^[27], Copyright © 2017 American Association for the Advancement of Science; (F) MIEC introduced a sulfur cathode to establish a spatially coupled ion-electron transport network; (G) The ion conductivity measurements at different temperatures. (F and G) are quoted with permission from Ref.^[30], Copyright © 2015 Springer Nature; (H) A trifunctional MIEC serves as a conductive binder for silicon anodes. This figure is quoted with permission from Ref.^[31], Copyright © 2024 ELSEVIER; (I) MIEC is coated onto the surface of active particles. This figure is quoted with permission from Ref.^[32], Copyright © 2026 ELSEVIER. HGF: Holey graphene framework; SEM: scanning electron microscope; MIEC: mixed ion-electron conductor; LFP: LiFePO₄; Go-Nb: graphene oxide Nb₂O₅; HGO: holey graphene oxide; GF: graphene framework; G: graphene; ASSB: all-solid-state batteries; SE: solid electrolytes; LPS: Li₂S·25P₂S₅; LP: lignin polymer; LiPAA: lithiated polyacrylic acid; PEDOT: poly(3,4-ethylenedioxythiophene); PSS: poly(styrenesulfonate); OMIEC: organic mixed ionic/electronic conductors.

To achieve this goal, two dominant design strategies have emerged. The first relies on spatially decoupled, dual-continuous ion/electron networks, in which ions and electrons are transported through independent yet percolating pathways, each optimized for its own transport requirements^[25]. This architectural decoupling mitigates local transport competition and suppresses reaction mismatch. Kuang et al. demonstrated such a strategy by assembling cellulose nanofibers and carbon black into a three-dimensional framework via electrostatic interactions^[26]. In this design, interconnected nanopores support ion diffusion while the conductive skeleton independently facilitates electron transport, achieving a volumetric energy density of 538 Wh·L^-1 at an ultrahigh loading of 60 mg·cm^-2 [Figure 2B]. Similarly, Sun et al. reported a Nb₂O₅/graphene framework, where a continuous graphene network enabled rapid electron delivery while hierarchical pores reduced ionic tortuosity [Figure 2C]^[27]. Strong interfacial coupling further minimized charge-transfer resistance, allowing the electrode to retain 54% of its capacity at 100 C and maintain high-rate performance even at increased areal loadings [Figure 2D and E]. These results demonstrate that spatially decoupled designs can outperform electrodes relying on single-pathway optimization.

The second strategy employs spatially coupled mixed ion-electron conductors (MIECs), which simultaneously transport ions and electrons within a single phase^[28]. By bridging ionic and electronic pathways at the molecular or mesoscale level, MIECs eliminate the intrinsic transport bifurcation of conventional electrodes, enabling rapid charge redistribution and reduced polarization. MIECs can function as conductive binders, active frameworks, or artificial interfacial layers, enhancing both ionic and electronic conductivity while stabilizing electrode interfaces^[29]. For example, the introduction of MIECs into sulfur cathodes has achieved room-temperature ionic conductivity of 10^-4-10^-2 S cm^-1 and electronic conductivity of 10^-3 S·cm^-1, resulting in high reversible capacity and excellent cycling stability [Figure 2F and G]^[30]. Notably, MIEC design priorities differ significantly between cathodes and anodes. Cathodes require high electronic conductivity and oxidative stability, whereas anodes demand balanced transport properties and mechanical robustness to accommodate volume changes and suppress lithium plating. Trifunctional MIECs developed for silicon anodes exemplify this approach, simultaneously providing electronic conduction, ionic transport, and mechanical reinforcement to preserve network integrity [Figure 2H]^[31]. MIECs can also serve as artificial interfacial coatings on active particles, further mitigating particle fracture and interface degradation [Figure 2I]^[32]. Moreover, MIECs can reduce polarization in full cells, enabling operation at lower negative-to-positive (N/P) ratios without inducing lithium deposition. Sun et al. significantly improved charge transport kinetics by constructing the MIEC through coating carbon black surfaces with red phosphorus^[33]. Even at N/P=1.15, the thick graphite anode effectively suppressed lithium deposition, whereas conventional thick electrodes exhibited pronounced dendrite growth.

Despite their promise, both spatially decoupled architectures and MIEC-based strategies face substantial challenges for industrial implementation. Decoupled designs require precise control over pore orientation, size distribution, and multiphase connectivity, posing challenges for process scalability. MIEC-based approaches depend on complex molecular design and multistep synthesis with finely balanced transport, mechanical, and electrochemical properties, limiting large-scale manufacturability and long-term reliability. Consequently, future thick-electrode development must strike a balance between structural sophistication and manufacturing feasibility, simplifying architectures and material systems while preserving essential ion-electron synergy to bridge the gap between laboratory demonstrations and practical deployment.

SUMMARY AND OUTLOOK

Thick electrodes offer a chemistry-agnostic pathway to simultaneously enhance energy density and reduce manufacturing costs in lithium-ion batteries by increasing active material loading and minimizing inactive components. However, thickness-induced ion transport limitations and fragile electronic networks often lead to severe capacity loss. Recent multiscale advances highlight ion-electron cooperative transport as a governing mechanism, demonstrating that electrode performance is dictated by the spatiotemporal matching of ionic and electronic fluxes rather than their independent maximization.

The integration of CIET theory with transport architectures based on spatially decoupled dual-continuous networks and spatially coupled mixed conduction pathways provides a mechanistic framework for next-generation thick electrode design. Despite significant progress, further advances are required in three key areas:

(1) Development of CIET-based models beyond conventional pseudo-two-dimensional (P2D) and Butler-Volmer frameworks;

(2) Scalable, cost-effective manufacturing strategies compatible with roll-to-roll processing, such as dry electrode fabrication with mechanically robust binder systems;

(3) Full-cell-level integration emphasizing capacity balancing and kinetic matching between thick cathodes and anodes.

Overall, harnessing coupled ion-electron transport is essential for overcoming the traditional energy-power trade-off and realizing high-energy-density battery systems.