Average Open Circuit Voltage Research Articles

Two-dimensional TTH-graphene: Structural, defective, and interfacial engineering on anode materials for potassium-ion batteries

Potassium-ion batteries (PIBs) demonstrate significant potential for future renewable energy storage systems, given the high natural richness and economic benefits of potassium resources. Nevertheless, the primary challenge hindering the development of PIBs is the scarcity of appropriate anode materials capable of delivering high performance. Using first-principles calculations, we theoretically design a two-dimensional graphene allotrope, termed TTH-graphene, constructed from assembled tetracyclo[6.6.0.02,6.010,14]tetradeca-1(8),2,4,6,9,11,13-heptaene (C14H8) structural units, which demonstrates good dynamical, thermal, and mechanical stability. The non-hexagonal symmetry can enhance the surface reactivity, making TTH-graphene a high-performance anode for PIBs with a low K migration barrier (0.22 eV), a moderate average open-circuit voltage (0.42 V), a high theoretical capacity (956.33 mA h g−1), and a small lattice expansion (1.2%). Furthermore, the appearance of vacancy defects enhances the K adsorption strength but induces the decrease in ionic diffusivity. Compared with the monolayer, the TTH-graphene bilayer exhibits an enhancement of the adsorption and diffusion performance of K on the outer surface.

First principle calculations of Janus 2D-TiSSe as an anodic electrode in batteries of lithium, sodium, and magnesium ions.

In recent years, rechargeable batteries have received considerable attention as a way to improve energy storage efficiency. Anodic (negative) electrodes based on Janus two-dimensional (2D) monolayers are among the most promising candidates. In this effort, the adsorption and diffusion of these Li, Na, and Mg ions on and through Janus 2D-TiSSe as anodic material was investigated by means of periodic DFT-D3 calculations. Electrochemical parameters were computed and compared. The diffusion barrier energies for migration of ions in monolayer TiSSe in three different potential directions were determined. Furthermore, the electronic properties and Mulliken charge analysis and plots of CDD were employed to investigate the interaction between ions and their surrounding surface. Our results show that the adsorption ability of TiSSe surface up to 32 metal ions falls in the following order: Li+ > Na+ > Mg2+. The maximum storage capacity is 337.37 mAh/g for Li/Na ion and 674.75 mAh/g for Mg ion. The average open-circuit voltage is 1.39, 0.93, and 0.73V for Li, Na, and Mg ions, respectively. Lastly, the minimum diffusion barriers follow the order Li+ < Na+ < Mg2+. The structural, energetic, and thermal stability of clean Janus surface and its saturated adsorbed systems was proved by MD simulations. In addition, we compared the obtained electrochemical parameters to those reported by other researchers. This comprehensive approach demonstrates valuable insights, furthering our understanding of TiSSe's behavior and its suitability for use in MIBs. DFT calculations were applied with projector augmented plane waves (PAWs)/PBE functional. A two-dimensional (2D) monolayer TiSSe surface was built with a 4 × 4 supercell. The energy cutoff of plane waves was set to 400eV. The DFT-D3 model has been used to incorporate van der Waals interactions. A geometric relaxation process was conducted using Monhkorst-Pack 8 × 8 × 1 in reciprocal space. The relaxation and electronic calculations were carried out using the Vienna Ab initio Simulation Package (VASP). Using the transition state (TS) search algorithm implemented in the Dmol3 module, linear synchronous transition and quadratic synchronous transit tools were utilized to find the minimum energy paths.

First-principles calculation of ZrS2 as anode material of aluminium ion battery

Developing low-cost, high-stability, and high-performance new ion battery anode materials is beneficial for the rapid development of ion batteries in fields such as rail transit, electronic products, and aerospace. In this work, the first-principles calculation method was used to study the feasibility of ZrS2 monolayer as an anode material for Al ion batteries. The Al ions adsorbed in the strain system tend to bind to the ZrS2 monolayer. In the adsorption system, the adsorption of Al increases the conductivity of the system. The theoretical specific capacity of Al under full load is 690.303 mAh/g. The average open circuit voltage (OCV) is calculated to be 0.635 V. The diffusion barrier of Al ions is 0.071 eV. ZrS2 monolayer is an attractive anode material for ion batteries.

DFT and AIMD studies of SnFe2O4 as a promising anode for Li-ion batteries

Tin ferrite (SnFe2O4) is considered as a perspective lithium ion battery anode, owing to its low cost, large theoretical capacity, low toxicity, structural stability, and easy synthesis method. Prior research has been done on the performance of lithium storage in SnFe2O4. However, the electrochemical processes that take place during the lithiation-delithiation cycle of LIBs have not been explained in depth yet. In order to understand the discharge mechanism in the LixSnFe2O4 anode (x = 0 to 2), we systematically investigated its electrochemical characteristics, structural and electronic properties, average formation energies, open-circuit voltages, diffusion coefficient, and volume expansion using density functional theory. According to our calculations, an increase in Li concentration x up to 1.125 leads to enhanced stability of the LixSnFe2O4 systems. During this process, Li+ ions prefer to be intercalated at the octahedral 16c sites, inducing the displacement of Sn2+ ions from tetrahedral sites 8a to 16c and 48f sites. However, for 1.125 < x < 2, lithium ions tend to occupy the less stable sites 48f , which reduces the stability of LixSnFe2O4 systems.Additionally, the calculated lithium intercalation voltage for full lithiation Li1.125SnFe2O4 is equal to 1.5 V, and the distortion of the LixSnFe2O4 system occurs at a voltage of 0.75 V, which is in well agreement with experimental results. The Li-coefficient diffusion on SnFe2O4 at 300 K is calculated by ab initio molecular dynamic simulation and equal to 8.22 × 10−8 cm2/s, which indicates the excellent mobility of Li ions in SnFe2O4. Considering all these results, we can suggest SnFe2O4 as a promising negative electrode material for lithium-ion batteries.

Exploring the potential of two-dimensional MB4 (M=Cr, Mo, and W) monolayer as a high-capacity negative electrode material for Al-ion batteries

Rechargeable metal-ion batteries can employ two-dimensional transition metal tetraboride monolayers because of their numerous accommodating sites, high specific surface area, and layered structure. Li-ion batteries (LIBs) can be efficiently replaced by aluminum ion batteries (AIBs) because aluminum is a cheap and abundant resource. In this work, the density functional theory approach has been used for Al-ion batteries to predict MB4(M= Cr, Mo, W) monolayers performance as anode material. We analyze these monolayers' structures, which are mechanically, thermally, and dynamically stable. Utilizing the energy-efficient adsorption of six Al-atom layers and their lightweight properties, with large storage capacities of 5066 mA h g−1, 3466 mA h g−1, and 2124 mA h g−1, with relatively low average open circuit voltages of 0.18 V, 0.15 V, and 0.11 V for CrB4, MoB4, and WB4, respectively, these monolayers show significant superiority over other 2D anode materials. Likewise, MB4 monolayer showed faster Al ion mobility as seen by the low activation barriers of 0.95 eV, 0.83 eV, and 0.49 eV for CrB4, MoB4, and WB4, respectively. Additionally, even after the full content of Al ions was adsorbed, the metallic characteristics of the materials were mostly retained. These traits suggest that monolayer CrB4, MoB4, and WB4 are promising anode materials for AIBs, with impressive rate capacities.

Insights into Na ion adsorption and diffusion in biphenylene as an anode material for sodium-ion batteries: A first-principles study

Anode materials are essential for the advancement of sodium-ion batteries (NIBs). This study comprehensively evaluates the biphenylene network (BP) as a promising anode material using first-principles calculations. Density functional theory (DFT) results reveal that sodium (Na) ions stably adsorb on BP surfaces, with adsorption energies ranging from −1.29 eV to −2.92 eV, due to effective charge transfer and hybridization between Na (s) and carbon (p) orbitals. The diffusion barriers for Na ion migration are 0.31 eV for the monolayer and 0.76 eV for the bilayer, with optimal paths involving the C8-ring and passing through C6- or C4-rings. Notably, edge sites were found to provide strong Na adsorption on monolayer BP nanoribbon, with low diffusion barriers (0.36 eV), revealing the critical role of edge configurations in enhancing the BP performance as an anode material. The theoretical capacity of Na on the BP monolayer is 908.52 mAh·g⁻¹, surpassing many other two-dimensional materials, and the average open circuit voltage (OCV) is 0.64 V. Overall, BP offers high Na storage capacity, low diffusion barriers, and suitable OCV, positioning it as a strong candidate for high-performance NIB anodes.

Theoretical design of graphene/tungsten dichalcogenide (Gr/WX2, X=S, Se) heterostructures used for anode materials of LIBs from DFT calculations

2D van der Waals heterostructures are highly promising for Lithium-ion batteries (LIBs), offering excellent energy capacity and extended lifespan when used as anodes. In this study, we systematically investigate the geometrical structures and electronic properties of graphene/tungsten dichalcogenide (Gr/WX2, X=S, Se) heterostructures, focusing on lithium (Li) atom adsorption and movement in these materials using first-principles calculations through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Our results reveal that the Gr/WX2 heterostructures possess excellent structural stability, with mechanical strength validated by their elastic constants. The diffusion barriers of Li atoms in Gr/WS2 and Gr/WSe2 heterostructures are competitive compared with other anode materials. Additionally, the Li diffusion coefficients at 300 K for Gr/WS2 and Gr/WSe2 are also surpassing those observed in pristine Gr. Furthermore, Gr effectively lowers the average open-circuit voltage (OCV) of Gr/WX2 heterostructures to optimal levels, with Gr/WS2 at 0.29V and Gr/WSe2 at 0.25V. The Gr/WS2 and Gr/WSe2 heterostructures after adsorption of Li exhibit remarkable thermal stability at 300K, maintaining their structural integrity throughout the simulation period. Our research provides comprehensive theoretical guidance for the practical development and application of Gr/WX2 heterostructures as anode materials in LIBs.

Theoretical study on the application of TaS2 as anode material of potassium ion battery in implantable medical equipment

The development of new energy storage equipment is of great significance to the development of implantable medical equipment. In this study, density functional theory (DFT) was used to study the two-dimensional nanomaterial TaS2 as anode material. It is found that potassium atoms can be adsorbed on the atoms of TaS2 substrate. The intrinsic TaS2 exhibits metallicity, and the shear deformation increases the band gap of TaS2 to 0.429 eV, which becomes a semiconductor property. In the O-doped TaS2 structure, the O-p orbital promotes the peak at the valence band. K shows different energy barriers on the surface of TaS2 and TaS2O, which are 0.289 and 0.243 eV, respectively. The theoretical capacity of K ion on TaS2 is 437.549 mAh/g. The average open circuit voltages of TaS2 and TaS2O are in the suitable voltage range.

Theoretical Investigation of a Novel Two-Dimensional Non-MXene Mo3C2 as a Prospective Anode Material for Li- and Na-Ion Batteries.

A new two-dimensional (2D) non-MXene transition metal carbide, Mo3C2, was found using the USPEX code. Comprehensive first-principles calculations show that the Mo3C2 monolayer exhibits thermal, dynamic, and mechanical stability, which can ensure excellent durability in practical applications. The optimized structures of Lix@(3×3)-Mo3C2 (x = 1-36) and Nax@(3×3)-Mo3C2 (x = 1-32) were identified as prospective anode materials. The metallic Mo3C2 sheet exhibits low diffusion barriers of 0.190 eV for Li and 0.118 eV for Na and low average open circuit voltages of 0.31-0.55 V for Li and 0.18-0.48 V for Na. When adsorbing two layers of adatoms, the theoretical energy capacities are 344 and 306 mA h g-1 for Li and Na, respectively, which are comparable to that of commercial graphite. Moreover, the Mo3C2 substrate can maintain structural integrity during the lithiation or sodiation process at high temperature. Considering these features, our proposed Mo3C2 slab is a potential candidate as an anode material for future Li- and Na-ion batteries.

Two-dimensional metallic Be2Al monolayer as a potential anode material for lithium-ion/sodium-ion batteries

Developing anode materials that combine excellent robustness, high-volume capacity, and superior electrical conductivity is essential for advancing high-performance ion batteries. Through comprehensive first-principles calculations, this paper illustrates the possibility of a two-dimensional Be2Al monolayer as an anode candidate for lithium/sodium-ion batteries. The Be2Al monolayer exhibits a high theoretical energy storage capacity (2382/2382 mAh/g for Li/Na) and an ultra-low diffusion barrier (124/77 meV for Li/Na). Its average open-circuit voltage (0.35/0.52 V for Li/Na) falls within the desired ideal range. Additionally, the Be2Al monolayer demonstrates excellent electrical conductivity, facilitating efficient electron transport. These promising outcomes indicate that the Be2Al monolayer can be an effective anode for next-generation lithium-ion and sodium-ion batteries.

Theoretical prediction on Irida-graphene monolayer as promising anode material for lithium-ion batteries

With the growing interest in energy storage technologies, the demand for alternative rechargeable batteries has become increasingly urgent. We evaluate the feasibility of a two-dimensional (2D) material, Irida-graphene (IG) monolayer, as an anode for lithium-ion batteries (LIBs) through first-principles calculations. IG monolayer, after Li adsorption, exhibits metallic property, suggesting the good electrical conductivity. Additionally, in comparison with IG bilayer, IG monolayer possesses low diffusion barriers (0.19–0.24 eV), high theoretical capacity (1115.8mA h/g), and low average open circuit voltage (0.317 V). Furthermore, under the influence of solvent effects, the adsorption performance becomes even more stable, and the theoretical capacity increases. These intriguing findings make IG monolayer great potentials for the anode of LIBs.

Computational prediction of two dimensional Mo4BC as promising anode material for lithium-ion batteries

The rising demand for high-capacity energy storage systems drives the hunt for cutting-edge lithium-ion batteries. Herein, calculations based on density functional theory have been performed to investigate the properties of two-dimensional (2D) Mo4BC monolayer as an anode material with high stability and storage capacity. Phonon dispersion and ab initio molecular dynamics (AIMD) simulations were performed to determine the dynamic and thermal stability, respectively. A low diffusion barrier of 0.12 eV supports the ultrahigh ionic mobility of Li ions and cycling capability for this material, which is essential for the fast charging/discharging process. In addition, the Li-ion adsorption on Mo4BC shows a high capacity of about 800 mAhg−1 with an average open circuit voltage (Vocv) of 0.03 V. Comprehensive overview of Mo4BC as high-capacity anode material with negligible low diffusion barrier and metallic character provides a significant contribution towards the advancement of energy storage systems such as lithium-ion batteries (LIBs). Our findings suggest Mo4BC as a promising anode material for high-performance LIBs, thus offering valuable insights for exploring innovative anode materials.

The first-principles study of 2D monolayer T-Mo2C as promising anode material for Lithium-ion Batteries

The growing demand for renewable energy technologies emphasizes the necessity for advancements in anode materials to enhance the performance of lithium-ion batteries (LIBs). In this research, we employ first-principles calculations to investigate the properties of a T-Mo2C monolayer proposed as an anode material. Phonon dispersion calculations and ab initio molecular dynamics (AIMD) simulations are conducted to assess the dynamical stability and thermal characteristics of the T-Mo2C monolayer as an anode material. Remarkably low diffusion barriers of 0.029 eV (Li) and 0.01 eV (Na and K) have been identified, facilitating rapid ion mobility. Furthermore, a monolayer of metal ions (M = Li, Na, K) absorbed onto T-Mo2C is explored, revealing a high specific capacity of approximately 1314.37 mAh/g (Li), 262.87 mAh/g (Na), and 32.85 mAh/g (K). The average open circuit voltage is determined to be 0.08 V (Li), 0.68 V (Na), and 2.59 V (K). This research provides a comprehensive overview of T-Mo2C monolayer as a high-capacity anode material, emphasizing its significant contributions to advancing energy storage systems, particularly in the context of LIBs. The findings presented here pave the way for the development of efficient and sustainable energy storage solutions to meet the escalating demands of the renewable energy landscape.

Theoretical evaluation of newly predicted VC4 monolayer for Li-ion batteries

Carbon-rich materials could increase the capacity and improve stability by decreasing the Li+-metal repulsive interactions. Herein, a new two-dimensional (2D) vanadium tetra carbide (VC4) was predicted using the first-principles calculations. The electrical characteristics, Li-anodic applications, and structural stability of VC4 were evaluated. With a comparatively less band gap, the bare VC4 displayed a semiconductor, while the Li+-loaded VC4 possessed a metallic nature. Using a multilayer Li top/bottom adsorption technique, a high Li storage capacity of 1353 mA h g−1 was observed in the VC4 monolayer. Notably, Bader's charge scheme confirmed a favorable and reversible electrochemical reaction between Li5VC4 and VC4. Significantly, VC4 serves as a host for high-performance lithium-ion batteries (LIBs), featuring a low barrier energy of 0.18 eV and an average open circuit voltage (OCV) of 0.52 V. Impressively, the variations of the structural parameters are within 10 % for a maximum Li+ intercalation, indicating high structural stability. These enthralling findings indicate that the monolayer VC4 sheet can be realized as an efficient Li host material for rechargeable LIBs.

First-principles study on the electrochemical properties of Na-ion-intercalatable heterostructures formed by transitional metal dichalcogenides and blue phosphorus

Extensive first-principles calculations have been performed to examine the electrochemical properties of Na-ion-intercalatable heterostructures formed by transitional metal dichalcogenides (MS2, where M = Ti, V, Nb and Mo) and blue phosphorus (BlueP), which have been reported as potential anode materials for rechargeable sodium-ion batteries. Upon formation of heterostructures, much improved structural stabilities have observed compared with the pristine MS2 and BlueP. Metallic T-TiS2, T-MoS2, H(T)-VS2 and H(T)-NbS2 would retain the conductive character after formation of heterostructures with BlueP, however, H-TiS2/BlueP and H-MoS2/BlueP would undergo a semiconductor to metallic transition accompanied by Na intercalation. Moreover, the presence of relatively low diffusion barriers ranging from 0.04 eV to 0.08 eV, coupled with the suitable average open-circuit voltage spanning from 0.12 eV to 0.89 eV, guarantee exceptional charge-discharge rates and ensure the safety of battery performance. Among these heterostructures, H(T)-NbS2/BlueP and T-TiS2/BlueP exhibit best Na adsorption ability of up to 4 layers, corresponding to theoretical capacities of 570.2 and 746.7 mAh/g, respectively. These encouraging properties indicate that T-TiS2/BlueP and H(T)-NbS2/BlueP could serve as suitable anode materials for high-performance sodium-ion batteries.

Theoretical investigation of NbB as an electrode material for Li/Na-ion batteries

The utilization of MBenes as electrode materials for lithium/sodium-ion batteries (LIBs/NIBs) has emerged as a prominent research area. The potential application of two-dimensional niobium boride (NbB) in LIBs/NIBs was explored through first-principles calculations. The results indicate that the monolayer of NbB exhibits exceptional dynamic stability, thermodynamic stability and electrical conductivity. The migration path of Li/Na on the NbB surface was determined, revealing diffusion barriers as low as 0.034 and 0.017 eV respectively, indicating the exceptional mobility and reversibility exhibited by NbB. The theoretical capacities of Li and Na on NbB are 258 and 323 mAh g−1, respectively. The open circuit voltage (OCV) varies within the range of 0.58 to 0.71 V and 0.05 to 0.81 V for Li/Na concentrations. Meanwhile, the investigation into the influence of various functional groups of NbB on the adsorption properties of Li/Na atoms reveals that both NbBO and NbBS surfaces retain their maximum capacity for Li/Na adsorption, whereas the NbBSe surface fails to achieve complete layer adsorption of Li/Na atoms. The introduction of the S functional groups not only maintains the ultra-low diffusion barrier (0.09 eV) of Na ions but also results in a significant decrease in the average OCV of Li/Na adsorption, which is approximately 0.37 and 0.14 V, respectively. The electrochemical properties of NbB in LIBs/NIBs are exceptional, positioning it as a promising electrode material within the MBenes family.

BC cone-shaped anodes for lithium-ion batteries

The efficiency of lithium-ion batteries (LIBs) depends upon anode materials possessing high capacity. In present study, we investigate boron carbide cone anode (BCC) for LIBs. The first-principles density functional theory (DFT) method is used to design anode materials based on BCC for application in LIBs. We found that the BCC shows negative Li adsorption energies. Our results also revealed the fast diffusions of Li ion on the BCC with the small energy barrier of 0.34 eV. The storage capacity of BCC system rank among the highest capacity of anodes for LIBs, with value of 785 mAh/g. The low average open-circuit voltage (OCV) value of BCC system indicates that this anode can handle a large operating voltage in LIBs.

TD-graphene: Theoretical prediction of a high-performance anode material for sodium-ion batteries with intrinsic metallicity, high capacity, and fast ion mobility

The utilization of pristine graphene as an anode material in sodium-ion batteries (SIBs) is limited by its inherent chemical inertness toward Na-ions. To address this issue, we propose a two-dimensional carbon allotrope (named as TD-graphene) by assembling tricyclo[4.4.1.11,6]dodecane (C12H20) skeleton. The topological non-hexagonal feature of C12H20 increases the degree of local carbon-ring disorder and introduces additional electron-deficient regions on the surface, thus enhancing the adsorption capability of Na. TD-graphene demonstrates exceptional stability across the energetic, thermodynamic, dynamic, and mechanical aspects. As a promising anode for SIBs, it exhibits an intrinsic metallicity, an ultra-high storage capacity (1487.58 mA h g−1), a low diffusion barrier (0.20 eV), a low average open-circuit voltage (0.33 V), and a small lattice expansion (0.6%). The presence of solvents with high dielectric constants improves the adsorption and migration capability of Na. Furthermore, taking into account the limitation of single-layer materials in practical applications, we employ h-BN as a promising substrate for TD-graphene, which can boost the Na adsorption and diffusion performance. These results render TD-graphene as a promising high-performance anode material for SIBs.

Theoretical investigation of Mo2C and Mo2CO2 as anodes for sodium/magnesium-ion batteries

Due to the rapid evolution of the global electronic product industry, the limited availability of lithium resources has prompted extensive research into alternative metal ion batteries that can substitute for lithium batteries. The exceptional potentiality for MXenes as electrode materials in energy storage batteries is demonstrated by excellent conductivity, expansive surface area, and mechanical strength. The performance of Na and Mg on Mo2C and Mo2CO2 monolayers have been investigated employing first-principles calculations, including geometry configurations, electronic structures, ion diffusion properties, open-circuit voltages, and theoretical specific capacities. The conductivity of stable anodes is superior both before and after ion adsorption. Additionally, the formation energies of Na/Mg on the monolayer are negative, indicating a strong binding between metal atoms and the substrate. The migration energy barriers of Na and Mg on Mo2C are estimated to be 0.017 eV and 0.070 eV, respectively, suggesting a significant level of mobility and reversibility for Mo2C. The predicted range of average open-circuit voltages for Na/Mg-ion battery anodes is approximately 0.05–1.00 V in the case of Mo2C and Mo2CO2. The highest concentrations of Na and Mg atoms on Mo2C and Mo2CO2 are achieved through multilayer adsorption up to Mo2CNa3.3, Mo2CMg2, Mo2CO2Na3.3, and Mo2CO2Mg1.8, resulting in corresponding theoretical capacities of 438, 526, 379 and 411 mA·h/g, respectively. In conclusion, the outstanding electrochemical performance of Mo2C and Mo2CO2 anode materials in sodium-ion batteries (SIBs) and magnesium-ion batteries (MIBs) has prompted the exploration of other MXenes electrodes with advantageous characteristics for ion battery applications, contributing to the advancement of renewable energy technology.

Adsorption of potassium atoms on twin T-graphene and twin-graphene surfaces for K-ion batteries

With the development of potassium-ion (K-ion) batteries, the search for two-dimensional materials with potassium storage potential has become a research hotspot. Twin T-graphene (TTG) and twin-graphene (TG) were studied as anode materials for K-ion batteries in terms of their adsorption energies, electronic structures, theoretical capacitances, diffusion barriers, and open circuit voltages (OCV) using first-principles calculations. Seven different adsorption sites of individual K atoms on TTG and TG were investigated. The most stable adsorption sites for TTG and TG were the H2 and B sites, respectively. After K atom adsorption, both semiconducting TTG and TG transform into metals, which facilitates K-ion diffusion and migration within the anode material. The K atoms were physically adsorbed on the TTG and TG surfaces, and the K adsorbed by TG was more stable than its TTG counterpart. Both TTG and TG could accommodate up to two layers of K atoms, with a maximum theoretical capacity of 372.22 and 496.30 mA·h/g, respectively. The average OCVs of TTG and TG were 1.12 and 1.58 V, while their diffusion barriers were 0.24 and 0.30 V, respectively. Therefore, TG has better theoretical capacity than TTG, while TTG excels in average OCV and diffusion barrier performance. This study underscores the broad potential applications of both TTG and TG as new energy materials for potassium storage.