Exploits, advances and challenges benefiting beyond Li-ion battery technologies

1. Introduction

• Growing electric-vehicle and energy-storage markets are expanding battery demand while Li-ion batteries remain inadequate for required energy density, cost-efficiency, and safety.Large-scale applications are constrained by lithium-resource insufficiency and safety factors.Source: §1 · p. 2
• Lithium supply may eventually run out, while offer–demand gaps could cause significant price fluctuations despite recycling and new sources.The introduction links this risk to projected large-scale electric-vehicle and smart-grid battery markets (Fig. 1a,b).Source: §1 · p. 2, Figure 1 · p. 3
• Li-ion batteries struggle with electric-vehicle range and cost and cannot readily meet grid-storage requirements for inexpensive production and output modulation.Adding battery stacks does not resolve long-range limitations or excessive costs, while some power plants can provide electricity at one-fifth the cost of available batteries.Source: §1 · pp. 2–3
• Sodium- and potassium-ion batteries use relatively abundant, inexpensive elements with lithium-like chemistry, but face low energy density, toxic and flammable electrolytes, high early-stage costs, and operational complexity.They therefore require further study toward low-cost, safer, long-cycle-life alternative chemistries.Source: §1 · p. 3
• The review surveys electrode materials and electrolytes for Na, Mg, Ca, Zn, and Al shuttles, halide anionic shuttles, NiMH and metal-hydride-accommodated Li-ion batteries, plus application-specific challenges and future research directions.It aims to compare relevant battery developments beyond classical Li-ion systems and provide perspectives for possible battery research directions.Source: §1 · p. 3

2.1.1. Motivation and current development

• Na-ion batteries offer greater sodium abundance and reduced thermal-runaway risk, while commercialization processes can adapt existing Li-ion technology; however, larger Na size and weight lower theoretical capacities.Source: §2.1.1 · p. 4
• Na-ion batteries are equally expensive to Li-ion batteries when costs are normalized to energy density and are not yet competitive with high-energy lithium-ion systems.They are expected to become complementary storage technologies depending on application requirements, with early deployment focused on stationary energy storage.Source: §2.1.1 · pp. 4–5

2.1.2.1. Cathode materials

• 185 mAh.g-1 at C/10 with 71% retention over 20 cycles and 70% after 100 cycles was delivered by α-NaMnO2, while electrolyte substitution improved long-term cyclability.The α-phase is more structurally stable than β-NaMnO2, and TEGDME-based electrolyte stabilizes interface resistance.Source: §2.1.2.1 · p. 5
• 200 mAh.g-1 with about 80% retention after 100 cycles was achieved using an ionic-liquid electrolyte, while Na(Ni0.5Mn0.5)O2 provides 125 mAh.g-1 at 2.2-3.8 V [11].The ionic liquid forms a uniform SEI layer, addressing electrolyte-related stability limitations.Source: §2.1.1 · p. 4, §2.1.2.1 · pp. 5–6
• NaNi0.5Ti0.5O2 delivers 121 mAh g-1 at C/5, retains 60% of initial capacity at 5C, and still provides 27 mAh g-1 at 10C.It exhibits reversible structural behavior, an average voltage of 3.1 V versus Na+/Na, and cyclability over 100 cycles at C/5 and 1C.Source: §2.1.2.1 · p. 6
• Other promising cathodes include Na2Fe2(SO4)3 at 3.8 V and 100 mAh.g-1, NASICON Na3V2(PO4)3 at 98.6% of theoretical capacity, and multi-cation oxides at 135 mAh g-1 with 99.7% efficiency over 250 cycles.Large diffusion channels, alluaudite tunnels, and ionic-liquid electrolytes support Na-ion transport and retention, although Mn dissolution and polysulfide-related challenges remain.Source: §2.1.2.1 · pp. 6–7
• Fluorophosphate Na1.5VPO4.8F0.7 reaches 600 Wh kg-1, with 95% retention after 100 cycles and ~84% after 500 cycles.The material exchanges 1.2 e-/f.u. at a 3.8 V working potential versus Na+/Na.Source: §2.1.2.1 · p. 8

2.1.2.2. Anode materials

• Graphite cannot intercalate the larger Na+ ion, so hard carbon is used as a host; alloying, conversion, metals, hydrides, and oxides are additional anode mechanisms under study.These alternatives complement Na+ intercalation as routes for accommodating sodium in anode materials.Source: §2.1.2.2 · p. 8
• Hard carbon is considered the most suitable SIB anode, with 413 mAh g-1 initial capacity and 90% retention over 50 cycles in 1M NaClO4 / EC:DMC.Earlier hard-carbon cells achieved 200 mAh g-1 with decent rate capability and cyclability over 180 cycles, while other carbonaceous structures remain under study.Source: §2.1.2.2 · p. 8

2.1.2.3. Electrolytes for SIB

• Electrolyte composition controls SIB interfacial reactions, SEI formation, charge-transfer resistance, and electrode-dependent performance, with NaPF6 improving SEI thermal stability.Adding DMC increased ionic conductivity by lowering mixed-solvent viscosity without significantly changing Na+ solvation; NaClO4 / EC:PC provided stable performance for Na4Fe3(PO4)2(P2O7) with hard carbon.Source: §2.1.2.3 · p. 8
• ~97 mAh g-1 reversible capacity over 120 cycles at 3.65 V with >98.5% coulombic efficiency was demonstrated for Na-ion full cells.The cells used EC0.45·PC0.45·DMC0.1 with hard carbon and a Na3V2(PO4)2F3 cathode, showing low polarization and good capacity retention.Source: §2.1.2.3 · p. 9
• Ionic liquids supported cathodes above 4.2 V with improved safety, whereas aqueous electrolytes offered greater safety but their 0–0.9 V window limited high-power and high-energy applications.Ionic liquids also reduced Mn dissolution at low voltages and provided low flammability, low volatility, broad electrochemical and thermal stability, low vapor pressure, and high ionic conductivity.Source: §2.1.2.3 · p. 9

2.2.1.1. Motivation, principle and historical development

• Mg anodes provide theoretical capacities of 2200 mAh g-1 and 3835 mAh cm-3, while Mg is abundant and safer to handle than Li.Mg abundance in Earth’s crust is at least four orders of magnitude higher than Li, and Mg generally does not form toxic or dangerous compounds.Source: §2.2.1.1 · p. 9
• Electrolyte development improved anodic stability from 1.5 V to 2.2 V and later to 2.5 V, but performance depends strongly on composition, solvent, additives, temperature, and electrodes.Dichloro-complex electrolytes demonstrated excellent reversibility over several thousand cycles, particularly with Chevrel cathodes.Source: §2.2.1.1 · pp. 10–11
• More than 2,000 cycles at 100% depth of discharge with less than 15% capacity deterioration were demonstrated using MgxMo3S4 cathodes and THF/Mg(AlCl2BuEt)2 electrolyte.Initial capacities were 60–90 mAh g-1 at 0.1–1 mA cm-2.Source: §2.2.1.1 · p. 10
• Chevrel-type cathodes have shown the best Mg2+ intercalation performance, but their specific capacity remains approximately 230 mAh g-1.Their advantages include Mg2+ intercalation kinetics, Coulombic efficiency, reversibility, and operational voltage.Source: §2.2.1.1 · p. 10

2.2.1.2.1. Cathode materials

• Chevrel-type Mo6S8 cathodes deliver approximately 120 mAh g-1 at 1.2 V with fast Mg2+ insertion, but show 20–25% room-temperature irreversibility from cation trapping.Mixed MgxMo6S6Se2 phases stored 110–100 mAh g-1 at 1.1–1.3 V after 100 cycles with DCC electrolyte and Mg anode.Source: §2.2.1.2.1 · p. 12
• Room-temperature Mg2+ intercalation in cathodes combining sufficient voltage and specific energy remains the largest obstacle, with spinel and layered materials requiring further improvement.Spinel diffusion barriers can be high, while layered compounds generally show low to moderate capacities and reversibility; none currently satisfies all functional-cathode requirements.Source: §2.2.1.2.1 · pp. 12–14
• 310 mAh g−1 at 50 mA g−1 and 192 mAh g−1 at 100 mA g−1 after 500 cycles were demonstrated by 2D VOPO4 nanosheets.The expanded interlayer spacing provides diffusion space for fast MgCl+ ion flux with low polarization.Source: §2.2.1.2.1 · p. 13
• Conversion cathodes can bypass slow Mg intercalation; Mg-S theoretically offers 1.77 V and 1,722 Wh kg−1, while engineered sulfur supports improved cycling.A ZIF-67-derived Co- and N-doped carbon support delivered ≈300-400 mAh g−1 after 150–250 cycles at up to 5 C, and Mg-I batteries exceeded intercalation-cathode energy density by ca. 400 Wh kg−1.Source: §2.2.1.2.1 · pp. 14–15

2.2.1.2.2. Anodes for Mg batteries

• Magnesium anodes have almost twice Li’s volumetric capacity and are generally dendrite-free, but MgO/Mg(OH)2 passivation inhibits plating and stripping.Conventional Li-battery electrolytes and O2 or H2O impurities at levels of 3 ppm can promote passivating layers.Source: §2.2.1.2.2 · p. 15
• Nanostructured Mg anodes reached 170 mAh g-1 with high reversibility, attributed to reduced passivation-layer thickness, while defective graphene was predicted at 1042 mAh g-1.A theoretical capacity of 1430 mAh g−1 was also predicted for Mg adsorption in arsenene at low voltage.Source: §2.2.1.2.2 · pp. 16–17
• Bi-based anodes combined substantial capacity with cycling stability, with Bi nanotubes delivering 350 mAh g-1 and 95-100 % Coulombic efficiency for 200 cycles.Bi alloying also produced 257-222 mAh g─1 over 100 cycles, whereas Bi0.88Sb0.12 delivered 298 mAh g─1 before declining to 215 mAh g-1 over 100 cycles at 1C.Source: §2.2.1.2.2 · p. 16
• MgNaTi3O7 full Mg-ion batteries delivered 75 mAh g−1 and 53 Wh kg−1, while alloy-anode electrolyte compatibility can impose higher equilibrium potential and lower specific capacity.The layered Na2Ti3O7/MgNaTi3O7/Mg0.5NaTi3O7 nanoribbon anode showed 78 mAh g−1 reversible Mg2+ insertion–extraction capacity.Source: §2.2.1.2.2 · pp. 16–17

2.2.1.2.3. Electrolytes for Mg batteries

• Mg electrolytes remain challenging because conventional polar organic electrolytes generally do not support reversible Mg deposition, while Grignard electrolytes are highly reducing and unstable toward electrophilic cathodes.DCC electrolytes reached 2.2V vs. Mg and 100% Coulombic efficiency, but their synthesis and electrochemistry depend sensitively on conditions and starting-material quality.Source: §2.2.1.2.3 · p. 17
• Electrolyte development improved Mg-battery reversibility, reaching 3.4 V oxidative stability, up to 100% Coulombic efficiency, and dendrite-free Mg plating in MAAC systems.Halides facilitate Mg diffusion and intercalation but are corrosive, motivating halide-free alternatives; Mg(BH4)2-based ionic-liquid electrolytes enabled reversible Mg deposition and stripping with 90% efficiency.Source: §2.2.1.2.3 · p. 18
• The reported electrolyte showed ionic conductivity around 1.8 mS cm−1, ~3.8 V stability, and 94% first-cycle Coulombic efficiency.Its cell’s discharge capacity fell from 180 to ca. 90 mAh g−1 after 10 cycles, improving on APC electrolyte deactivation after the first cycle at high voltage.Source: §2.2.1.2.3 · pp. 18–19
• Adding (NH4)+ ions to Mg(BH4)2 solutions improved Mg2+ ion mobility.Source: §2.2.1.2.3 · p. 19
• Magnesium scandium selenide spinel showed ~0.01–0.1 mS cm–1 Mg-ion mobility at 298 K, suggesting chalcogenide spinels could enable solid ionic conductors.The proposal supports all-solid-state magnesium batteries.Source: §2.2.1.2.3 · p. 19

Ca-ion batteries: State-of-the-art

• Progress remains limited by slow Ca2+ diffusion, electrolyte reduction, passivation layers, and the difficulty of identifying compatible cathodes, electrolytes, and stable chemistries.Reversible calcium deposition previously required elevated temperatures near 100 °C, and development of rechargeable Ca batteries remains unlikely without creative approaches [189].Source: §2.2.2 · p. 20
• Calcium anodes offer 2072 mAh cm−3 and 1337 mAh g−1, compared with 300–430 mAh cm−3 and 372 mAh g−1 for graphitic Li-ion anodes.Calcium’s abundance, lower charge density, and safety advantages have motivated calcium-battery research.Source: §2.2.2 · p. 20
• Room-temperature Ca plating and stripping became feasible in Ca(BH4)2/THF, delivering 1 mAh cm−2 at 1 mA cm−2, polarization near 100 mV, and more than 50 cycles.A small amount of CaH2 formed and protected the calcium metal anode at open circuit.Source: §2.2.2 · p. 21
• A reversible Ca full battery using carbon-based electrodes achieved 66 mAh g−1 at 2 C, 4.6 V, 94% retention after 300 cycles, and a final capacity of 62 mAh g−1.The chemistry reversibly plated and stripped Ca2+ from carbonate Ca(PF6)2 electrolyte while co-intercalating PF6− in the cathode.Source: §2.2.2 · p. 21
• Ca2+ intercalation reached ~450 mAh g−1 in crystalline V2O5 and 465 mAh g−1 in a vanadate nanocomposite, while propylene carbonate improved performance two- to tenfold over conventional V2O5.A reported capacity increased from 60 to 80 mAh g−1 after the 50th cycle at an average voltage of 2.6 V.Source: §2.2.2 · p. 21
• A Ca[B(hfip)4]2 electrolyte reversibly plated and stripped calcium at room temperature, with oxidative stability up to 4.5 V and ionic conductivity above 8 mS cm−1.Its weakly interacting, large fluorinated anion enhances calcium-ion mobility, and the electrolyte is compatible with multiple host materials.Source: §2.2.2 · pp. 22–23

Overview of Zn batteries

• Aqueous ZIBs offer ~1 S cm-1 ionic conductivity versus ~0.01 S cm-1 for non-aqueous electrolytes and potential grid-scale applicability.Their appeal is linked to avoiding inert atmospheres and toxic, flammable carbonate electrolytes while reconsidering cost, safety, cycle life and recycling.Source: §2.2.3 · p. 23
• Aqueous ZIBs use Zn anodes with manganese-, vanadium-based oxides, Prussian blue analogs and polyanion cathodes, but alkaline Zn–MnO2 systems suffer Zn dendrites and irreversible discharge capacities.Storage commonly involves Zn2+ insertion/extraction in tunnel- or layered-structure compounds under slightly acidic aqueous conditions.Source: §2.2.3 · p. 23
• In ZIBs, V-based cathodes offer greater stability and various V oxidation states than Mn-based cathodes, while Zn-ion insertion depends on the electrolyte solvent and salt concentration.Source: §2.2.3 · pp. 23–24
• Hydrated Zn0.25V2O5·nH2O facilitates Zn2+ intercalation through an expanded structure and reversibly exchanges 1.1 Zn2+ to form Zn1.35V2O5·nH2O.Its electrochemical mechanism remains complex and may involve multistep reactions.Source: §2.2.3 · p. 24
• Crystal water stabilizes the host structure, while Zn2+/H+ insertion preserves V2O5 interlayer spacing and benefits long-term cycling stability.The mechanism can also involve multistep phase transitions, including tunnel-to-layered transformations caused by structural expansion.Source: §2.2.3 · pp. 23–25
• 426 mAh g-1 at 0.2 A g-1, ~268 Wh kg-1 at 1400 W kg-1, and 96% capacity retention after 20,000 cycles were achieved by a hydrated Zn0.3V2O5·1.5H2O aqueous battery.The design used a hierarchically porous cathode, 3M Zn(CF3SO3)2 aqueous electrolyte and Zn anode; phase-transformed nanoflowers increased contact and active Zn-storage sites.Source: §2.2.3 · pp. 24–25

2.2.4. Al batteries: Electrolyte challenges

• Al batteries achieve high power and up to 7,500 cycles, but remain early-stage technologies with low energy density from difficult Al3+ intercalation.Their application domain is restrained and initial cost remains high; room-temperature ionic liquids are non-flammable but costly.Source: §2.2.4 · p. 25
• The first Al-ion rechargeable battery used V2O5 nanowire cathodes and Al metal, delivering a 0.55 V intercalation plateau and 305 mAh g-1 initially versus 273 mAh g-1 after 20 cycles.The electrolyte was ionic-liquid based.Source: §2.2.4 · p. 25
• Fluorinated graphite delivered approximately 300 mAh g-1, but its coulombic efficiency was only 75% over 40 cycles.A 3D graphitic-foam cell also showed discharge plateaus near 2 V and cycling up to 7,500 cycles at ultrahigh current densities.Source: §2.2.4 · p. 25
• An aqueous Al battery rapidly charged and discharged, operating at 1.35 V charge and 1.0 V discharge with 60 mAh g-1 at 2 A g-1.Using graphite nanosheets and zinc in aqueous electrolyte, it maintained over 200 charging/discharging cycles at low cost.Source: §2.2.4 · p. 25
• Aqueous systems require highly concentrated Al salts for high energy density, causing acidity through Al3+ hydrolysis and corrosion of the zinc negative electrode.Compared with ionic-liquid batteries, aqueous electrolytes offer high working voltage, stable cycling, and lower cost.Source: §2.2.4 · pp. 25–26

3.1. F-ion battery: a novel solid-state battery

• Fluoride ions shuttle reversibly between metal electrodes, where metal-halide formation-energy differences generate electromotive force and enable theoretical voltages of 1–3 V.Multiple metal oxidation states and high metal-fluoride densities can support high theoretical energy densities.Source: §3.1 · p. 26
• The first reversible fluoride-ion battery used a BiF3 cathode, Ce anode, and tysonite electrolyte, but required 160°C because conductivity was only 10^-7 S cm^-1 at room temperature.The electrolyte was 600 µm thick for mechanical stability, motivating interface, thin-film, and sintered-material improvements.Source: §3.1 · p. 26
• 120 mAh g–1 was delivered at room temperature by the Sn/BaSnF4/BiF3 fluoride-ion cell at 10 μA cm–2, with cycling demonstrated from RT to 150°C.The cell used ternary-fluoride electrolytes with ionic conductivities on the order of 10^-1 S cm^-1; Fig. 8 reports its voltammetry and cycling performance.Source: §3.1 · pp. 26–27, Figure 8 · p. 27
• Fluoride-ion liquid-electrolyte secondary batteries remain unconvincing because proton-driven HF formation and poor reversibility cause rapid fading after a few cycles.The room-temperature cell’s 120 mAh g–1 capacity was below BiF3’s 302 mAh g–1 theoretical capacity because conversion to Bi was incomplete.Source: §3.1 · pp. 26–27

3.2. Cl-ion battery: Early stage of development

• Chloride-ion batteries offer theoretical energy densities above current LIBs, abundant electrode materials, and room-temperature operation at rates up to 2C.Liquid electrolytes can transport chloride ions using suitable ionic-liquid mixtures, but chloride solubility limits electrode choices such as FeCl3.Source: §3.2 · pp. 27–28
• Chloride solid electrolytes generally require high temperatures for fast ion mobility, although cubic CsSnCl3 reaches 1 mS cm-1 at 100°C and needs further battery evaluation.Low-solubility chloride hosts and oxychlorides are proposed to address electrolyte dissolution.Source: §3.2 · p. 28

4.1. NiMH batteries

• NiMH batteries are cheaper than lithium batteries, safer during overheating, support high-rate charge and discharge, and operate without thermal runaway in railway systems.Panasonic cells store about 1.3 kWh at approximately 30–40 kg with an 8-year lifetime, while Kawasaki systems provide stable 3C discharge, 20C peak current, and regenerative braking.Source: §4.1 · p. 28
• NiMH cells use concentrated KOH electrolyte, with Ni(OH)2/NiOOH cathode and reversible M/MHx metal-hydride anode reactions during discharge.The reported standard electrode potentials are +0,49 V for the cathode and -0,83 V for the anode.Source: §4.1 · p. 28
• Mg-containing ternary materials reduce amorphization induced by repeated hydrogenation of binary phases and stabilize multi-plateau pressure behavior into a single plateau in the practical pressure range, with capacity enhancement up to about 400 mAh g-1.Source: §4.1 · p. 29
• Mg-H2 systems offer major capacity potential but remain limited by slow hydrogenation, Mg corrosion, and stable MgH2; AB2 and V-based alloys also face activation and cost issues.Nanoscaled Mg-based materials, surface engineering, and LPSO phases are proposed to improve performance, while AB2 and V-based compounds reach 400 mAh g-1 but can activate poorly.Source: §4.1 · p. 30
• Zr- and Ti-containing C15/C14 Laves alloys exceed 420 mAh g-1, with rapid-solidification processing and 3% LaNi catalysis improving hydrogen diffusion and activation.Figure 10 reports a maximum discharge capacity of 420 mAh g-1 for Ti0.2Zr0.8La0.03Ni1.2Mn0.7V0.12Fe0.12.Source: §4.1 · pp. 30–31, Figure 10 · p. 30
• Rare-earth-free TiFe can theoretically provide as high as 500 mAh g-1 but is not electrochemically active in alkaline media due to surface passivation, whereas TiNi has excellent electrocatalytic activity and corrosion resistance but delivers 150 mAh g-1 at C/10.Source: §4.1 · p. 31
• Replacing 20% of Ni by Cu doubles TiNi hydride reversible capacity to 300 mAh g-1 at C/10, with good kinetics up to C/2.TiNi0.8Cu0.2 electrodes also show easy activation.Source: §4.1 · p. 31

4.2. LIBs accommodating metal hydrides as anodes and electrolytes

• Metal-hydride conversion anodes offer low potential, low polarization, and very high capacities, with capacity governed by the metal molar mass and hydrogen content.The conversion is MHx + xLi+ + xe-  M + xLiH (3).Source: §4.2 · p. 32
• Mg-based hydrides are attractive because Mg is light, abundant, and inexpensive, but their favorable conversion reaction is kinetically sluggish.Nanostructuring MgH2 can help overcome this limitation.Source: §4.2 · p. 32
• A 1 μm MgH2 film lithiat​ed fully on the first cycle but achieved only 25% delithiation, attributed to poor room-temperature mass transport despite increased conductivity from metallic Mg.The film thickness doubled while cohesion was preserved.Source: §4.2 · p. 32
• In MgH2–TiH2 composites, Li extraction depends strongly on composition, ranging from hydride reformation to partial TiH2 reversibility or full irreversibility.A cooperative Mg/LiH matrix effect improves TiH2 reversibility and lowers polarization through modified interfaces and better volume-change adaptation.Source: §4.2 · p. 32
• Li-doped NiTiH systems show practical improvements in electrochemical capacity, voltage, and volume change and are predicted as suitable LIB anodes.Pure and Li-doped NiTiH were studied using DFT and MD calculations.Source: §4.2 · p. 33
• Solid-state LiBH4 electrolytes operated from room temperature to 120°C improve hydride-anode reversibility and cycling, supporting safer high-capacity, high-energy LIBs than carbonate liquids.The solid-state approach avoids issues associated with liquid electrolytes during voltage solicitation.Source: §4.2 · p. 33

5.1. Cationic shuttles

• SIBs can approach classical LIB performance, reaching ~300 Wh kg-1 with a 210-cycle life, but their energy density with similar electrodes remains lower than LIBs for electric vehicles.Source: §5.1 · p. 33
• Mg batteries have demonstrated prototype cells, but compatible electrode–electrolyte chemistries must safely conduct Mg2+ with acceptable cyclability and higher-voltage stability.Current electrolytes often limit charging to around 3 V or 1.5 V, while halides and fluorides can corrode or explosively affect components and aluminates are air- and water-sensitive.Source: §5.1 · p. 34
• Mg electrode optimization remains difficult because intercalation cathodes trade capacity for stability, conversion cathodes trade energy density for cycling, and passivation or dendrites impair Mg anodes.Solid magnesium ionic conductors and defined dendrite-free plating/stripping windows are suggested research directions, while replacing metallic Mg can reduce the theoretical capacity advantage.Source: §5.1 · p. 34
• Cross-system performance comparisons are difficult because studies use different electrolytes, electrodes, cell combinations, and unreliable pseudo-reference electrodes, motivating consistent half-cell protocols.Reported performance reflects the overall cell combination, making individual-component conclusions unreliable.Source: §5.1 · p. 35
• Ca-ion batteries generally exhibit working voltages below 2.0 V and cycling within 100 cycles because slow polyvalent-ion diffusion and Ca passivation hinder reversible operation.Hydrated hosts, aerogels, large weakly binding anions, and intercalation-type anodes are proposed to improve diffusion or avoid calcium plating and stripping.Source: §5.1 · p. 35
• ZIBs are estimated at 85 Wh kg-1 with Mn-based cathodes and 75 Wh kg-1 with V-based cathodes, comparable to Ni-MH but below LIBs at 180–230 Wh kg-1.Despite the lower energy density and limited aqueous voltage window, safety, cost, environmental benefits, extreme-temperature operation, and flexible quasi-solid-state demonstrations support targeted applications.Source: §5.1 · pp. 35–36
• Flexible hybrid ZIBs using Pluronic hydrogel electrolytes reached 1.8 V for Zn/PHE/LMO and 1.18 V for Zn/PHE/LFP versus Zn2+/Zn, enabling wearable and self-charging solar-integrated demonstrations.The hydrogel used 0.25M ZnSO4 and 0.25M Li2SO4 in an aqueous mixed-salt solution.Source: §5.1 · p. 36
• Al-ion rechargeable-battery research is comparatively recent, with the first rechargeable Al-ion battery reported in 2011 after earlier emphasis on primary batteries.Advances in material nanostructuring helped shift attention toward rechargeable Al-ion systems.Source: §5.1 · p. 36
• An Al-ion battery using an ionic-liquid electrolyte demonstrated 7500-cycle stability at 2 V versus Al3+/Al, while acidity, impurities, cost, low energy density, and electrochemical performance remain challenges.Aqueous Al-ion batteries have lower voltage and energy density but could suit stationary energy-storage systems; dual-graphite batteries use graphite as an intercalation material.Source: §5.1 · p. 36

5.2. Anionic shuttles

• Solid-state F-ion and Cl-ion batteries show their best cycling at 150 °C, while ongoing work targets higher energy density, better cyclability, and lower operating temperature.A Li–FeOCl cell delivered 158 mAh g-1 initially and 60 mAh g-1 after 30 cycles, limited by large charged–discharged volume changes.Source: §5.2 · pp. 36–37
• An all-solid-state Cl-ion battery improved working voltage and capacity retention over 20 cycles using a solid polymer electrolyte.Source: §5.2 · p. 37

5.3. MH-based batteries

• Ni-MH batteries combine aqueous-electrolyte safety, low cost, robustness, and high specific power, supporting strong HEV adoption, but remain inferior to Li-ion for full-EV energy density.Figure 11 compares commercial batteries and shows Li-ion exceeding Ni-MH in both gravimetric and volumetric energy density [317].Source: §5.3 · p. 37, Figure 11 · p. 37
• Alternative metal hydrides raise reversible capacity from A2B4/AB5 alloys at 1.5 wt.% and 400 mAh g-1 to MgH2 at 7.6 wt.% and 2000 mAh g-1.Pd-capped Mg-Sc and Mg-Ti thin films achieved capacities up to 1500 mAh g-1, but electropositive hydrides are more corrosion-prone in aqueous KOH.Source: §5.3 · p. 38
• MH-based battery development must balance higher energy density against corrosion and safety: coatings or lower-corroding electrolytes are proposed, whereas organic electrolytes enable higher voltage but sacrifice aqueous Ni-MH safety.Efficient low-cost coatings for current 3D electrodes and suitable low-corroding electrolytes remain challenging, while aqueous-electrolyte safety continues to motivate research.Source: §5.3 · p. 38

• Table 2 compares beyond Li-ion batteries by energy density relative to LIBs and durability, while Fig. 12 summarizes electrode- and full-cell energy density.Some full-cell data are available only at high current rates, which may affect their specific energy.Source: §6 · p. 42
• Sodium-ion batteries are approaching commercial LIB energy-density performance, but their long-term lifespan still requires improvement.Academic and start-up development may reduce prices compared with LIBs, while large-scale production and new positive electrodes could improve performance.Source: §6 · p. 42
• Beyond-Li-ion development includes Mg, Ca, Zn, and Al multivalent-cation systems, whose success depends on electrode optimization, cell configuration, and assembly engineering.Suitable cell integration is required to exploit materials that may surpass state-of-the-art LIBs.Source: §6 · pp. 42–43
• Solid-state or gel electrolytes are being pursued for safer operation, higher energy density, and long stability, while aqueous Zn and Al batteries may serve quasi-stationary storage.NiMH batteries continue supplying rechargeable sectors, with further integration into electric vehicles and railways.Source: §6 · p. 43
• Metal-hydride-integrated Li-ion batteries demonstrate potential as solid-state Li/Na batteries combining high energy density and safety.This MH-LIB configuration is identified mainly for future applications.Source: §6 · p. 43
• Post-Li-ion assessment must specify end-of-life energy density and examine interface stability, degradation mechanisms, and long-term operation to enable better performance and lower recycling costs.These studies should cover varied operating conditions and support suitable treatments and creative remedies.Source: §6 · p. 43

Comparative scope

• Table 1 compares battery technologies using cathode capacity, cycling stability or Coulombic efficiency, full-cell energy density, advantages, and drawbacks.The comparison spans cathodes, anodes, electrolyte compositions, operating temperatures, and assessed full-cell performance.Source: Table 1 · p. 60, Graphical Abstract · pp. 60–61

Li-ion benchmark

• Li-ion full cells provide 90-120 Wh kg-1 with 2,000 cycles and LiCoO2 cells provide 150-240 Wh kg-1 with 500-1000 cycles, combining high energy density with cost and safety drawbacks.The Li-ion entries are presented as benchmarks for comparison with beyond-Li-ion systems.Source: Graphical Abstract · p. 61

Na-ion batteries

• Na-ion systems offer 80-100 Wh kg-1 and 120-210 cycles, with abundance, low large-scale cost, better safety, and wide-temperature operation but moderate energy density.A Na-ion cathode entry also reports 70-114 Wh kg-1 and 100 cycles, with ionic-liquid cost noted as a drawback.Source: Graphical Abstract · p. 61

Mg and Ca batteries

• Mg and Ca systems show 38-45 Wh kg-1 with 120-2,000 cycles and 40-45 Wh kg-1 with 300 cycles, respectively, emphasizing abundance and low cost.The Mg entry notes low capacity, while the Ca entry highlights low-temperature operation; related Ca performance is 16-20 Wh kg-1 over 50 cycles.Source: Graphical Abstract · p. 61

Zn-ion batteries

• Zn-based aqueous batteries combine 70-75 Wh kg-1 with 4,000 cycles or 60-85 Wh kg-1 with 20,000 cycles, supporting safety, low cost, and facile manufacturing.An additional aqueous Zn configuration reports 15-25 Wh kg-1 and 200 cycles for large-scale stationary storage.Source: Graphical Abstract · p. 61

Other battery families

• The abstract also covers Al-ion, NiMH, MH-based Li-ion, and F-ion systems, spanning high power or energy density advantages alongside low voltage, acidity, operating-temperature, and capacity-fading limitations.Reported examples include Al-ion at 35-40 Wh kg-1 and 7500 cycles, NiMH at 40-120 Wh kg-1 and 300-500 cycles, and F-ion below 40 Wh kg-1 with 10-50 cycles.Source: Graphical Abstract · p. 61