Review Article 
Corresponding author: Anna O. Diteleva ( anna.diteleva@mail.ru ) Corresponding author: Alena V. Popkova ( popkovaalena@rambler.ru ) © 2023 Vladimir V. Sleptsov, Lev V. Kozhitov, Anna O. Diteleva, Dmitry Yu. Kukushkin, Alena V. Popkova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Sleptsov VV, Kozhitov LV, Diteleva AO, Kukushkin DYu, Popkova AV (2023) Recent progress and development prospects of mobile current sources. Modern Electronic Materials 9(2): 7790. https://doi.org/10.3897/j.moem.9.2.109923

Physicochemical fundamentals have been developed for the basic design solutions and fabrication technologies of prospective electrolytic power cells with a reusable cell capacity of 350–500 W·h/kg at the first stage and 1000 W·h/kg at the second stage. Along with conventional chemical current sources and ionistors, there are emerging highperformance supercapacitor structures with thin dielectric in the double electric layer and hybrid capacitors in which energy is accumulated in the double electric layer and due to electrochemical processes. This approach reduces the internal resistance of the electrolytic cells thus decreasing the heat emission during operation and therefore providing for a higher specific energy capacity and operation safety, shorter charging time and an increase in specific power. Prospective anode is a nanostructured electrode material in the form of a carbon matrix filled with a nanostructured chemically active material. Promising carbon matrix fillers are Li and its alloys, Si, Al, Na, Sn, Mg, Zn, Ni, Co, Ag, as well as a range of other materials and their compounds. The effect of carbon material specific surface area, dielectric permeability and chemically active material addition on the specific energy capacity has been studied. Theoretical specific energy capacity of metal/air hybrid capacitors has been calculated. Thinfilm technological system has been designed for new generation electrode materials in the form of carbon matrices with highly developed surface containing thin tunneling dielectrics and chemically active materials on dielectric surface.
hybrid capacitor, current sources, energy storage devices, carbon material, electrode materials, nanustructuring, nanoparticle, carbon matrix, chemically active material
Sustained development of electrically driven vehicles, individual power supply systems for residential and industrial buildings, security systems and other applications requires, according to experts, reusable power sources with a specific energy capacity of 350–500 W·h/kg at the first stage and 1000 W·h/kg at the second stage. The highest capacity power sources manufactured by thickfilm technologies are currently lithium chemical current sources (LCS) the highest energy capacity of which reaches 260 W·h/kg [
Energy capacity of high specific surface area carbon matrix CCS with silicon nanoparticles as a function of number of cycles
Thus a promising CCS anode design is a nanostructured electrode material in the form of a carbon matrix filled with a nanostructured chemically active material. Since an elastic matrix has a high specific surface area, energy accumulation in the electrode material occurs by two mechanisms (due to electrochemical reactions and in the double electric layer (DEL)). As a result the electrochemical cell for 3–5generation current sources is conceived to be a hybrid capacitor. Further CCS specific energy capacity growth to 500 W·h/kg and higher is associated, according to the roadmap, with the development of metal/sulfur and metal/air CCS in which the anode is connected to the cathode delivering sulfur or oxygen, respectively. The electrode materials for the CCS will be high specific surface area carbon matrices filled with functional materials in the form of chemically active and auxiliary materials. Promising carbon matrix filler materials are Li and its alloys, Si, Al, Na, Sn, Mg, Zn, Ni, Co, Ag and a number of other materials and their compounds [
Since the research into current sources implementing DEL and electrochemical process based energy accumulation mechanisms has led to the development of multiple electrochemical cell design options (CCS, ionistors, highperformance supercapacitors with thin tunneling dielectric in DEL and hybrid capacitors), there is the necessity to develop physical and mathematical models describing the basic design, material science and technical solutions of the prospective element base.
Theoretical analysis of energy accumulation in electrode materials of hybrid capacitors based on a thermodynamical approach implying the summation of all energy types in the system in question allows writing the energy balance equation for the hybrid capacitor as follows:
$\frac{C{U}_{\mathrm{c}}^{2}}{2}+\sum {M}_{i}{N}_{i}={I}_{\mathrm{L}}{U}_{\mathrm{L}}t+{I}_{\mathrm{c}}^{2}{R}_{\mathrm{ESR}}t$, (1)
where C is the capacity, U_{c} is the capacitor voltage, M_{i}N_{i} is the product of chemical potential and the number of particles, I_{L} is the load current, U_{L} is the load voltage, t is the charging/discharging time, I_{c} is the capacitor current and R_{ESR} is the electrical resistance of the capacitor structure.
Equation (1) describes the ideal capacitor free of leakage currents. To simplify the problem one can consider the operation of that capacitor for minimum charging/discharging time. Simple transformations of Eq. (1) yield the following equation of the capacitor energy Е_{c}:
${E}_{\mathrm{c}}=\left(\frac{\epsilon {\epsilon}_{0}}{2d}{U}_{\mathrm{c}}^{2}+i{U}_{\mathrm{c}}t{I}_{\mathrm{c}}^{2}\frac{{R}_{\mathrm{ESR}}}{{S}^{2}}t\right)S$, (2)
where ε is the dielectric permeability, ε_{0} is the relative dielectric permeability, i is the electrochemical reaction rate, U_{c} is the capacitor voltage, t is the charging/discharging time, I_{c} is the capacitor current, R_{ESR} is the electrical resistance of the capacitor structure, S is the surface area and d is the double electric layer thickness.
The electrochemical reaction rate (in A/cm^{2} or A/m^{2}) is commonly related to a specific surface area and defined as current density:
$i=\frac{I}{S}$.
It can be seen that the specific energy capacity of a hybrid capacitor is determined by the sum of the interrelated parameters R_{ESR}, S, U_{c}, I_{c} and t.
Equation (2) describes the ideal hybrid capacitor free of leakage currents. To simplify the task one can consider the operation of that capacitor for minimum charging/discharging time.
FractalCalculation software was designed for capacitor parameter calculation in Python programming language, code UTF8. Python is one of the most widely used crossplatform programming languages. FractalCalculation incorporates two blocks with software code describing software operation principle: 1) data calculation, acquisition and displaying algorithm and 2) UI (user interface) software code or, simply, what the user works with.
Based on the mathematical model developed, the specific energy capacity of prospective electrolytic cell designs was calculated: 1) ionistors, i.e., highperformance supercapacitors the specific energy capacity of which is determined, primarily, by the specific surface area of the electrode materials; 2) highperformance supercapacitors with thin tunneling dielectric in the double electric layer, the dielectric permeability of which can be varied over a wide range; 3) hybrid capacitors in which energy is accumulated in the DEL and due to chemical reactions; 4) hybrid capacitors with thin tunneling dielectric in the DEL, the dielectric permeability of which can be varied over a wide range and energy is accumulated in the DEL and due to chemical reactions.
The specific energy capacity of ionistors is determined, primarily, by the specific surface area of the electrode materials. The effect of the specific surface area of the electrode material (300–3000 m^{2}/g) on the specific energy capacity of cells was studied for a constant dielectric permeability using the energy balance equation and the software designed. The unknown parameter for the calculation was the thickness d for water and polymer electrolyte cells. Table
Cell #  Busofit fabric area (cm^{2})  Weight of Busofit fabric with preset dimensions (g)  Outer surface area (m^{2})  Charging / discharging voltage (V)  Charging / discharging current (A)  Time (s)  Capacity (F)  Energy capacity (W·h/kg)  ESR (Ohm)  Weight (g)  Softwarecalculated parameters  
d (nm)  i  
Water electrolyte cells (dielectric permeability ε_{wat} ~ 80)  
1  60  1.43  1716  2.5  0.15  865  87  4.2  1.47  18  13.95  0 
2  81  1.93  2316  2.5  0.15  2000  141  4.9  0.315  25  11.61  0 
3  400  9.66  11592  2.5  0.15  6496  521  4.5  0.457  100  15.83  0 
Polymer electrolyte cells (dielectric permeability ε_{polym} ~ 8)  
1  75  1.8  2160  4  0.5  460  151  16  1.0  20  1.1  0 
2  192  4.61  5532  4.5  0.5  1200  355  21  1.5  46  0.9  0 
3  340  8.1  9720  3.5  0.5  2400  570  12  0.3  80  0.6  0 
The specific energy capacity of the cells (E_{sp}, W·h/kg) was calculated using the formula
${E}_{\mathrm{sp}}=\frac{E}{3600m}=\frac{\left(\frac{\epsilon {\epsilon}_{0}}{2d}{U}_{\mathrm{c}}^{2}+i{U}_{\mathrm{c}}t{I}_{\mathrm{c}}^{2}\frac{{R}_{\mathrm{ESR}}}{{S}^{2}}t\right)S}{3600m}$, (3)
where m is the cell weight.
Since the calculation is conducted for capacitors, iU_{c}t = 0 and Eq. (3) can be rewritten as follows:
${E}_{\mathrm{sp}}=\frac{E}{3600m}=\frac{\left(\frac{\epsilon {\epsilon}_{0}}{2d}{U}_{\mathrm{c}}^{2}{I}_{\mathrm{c}}^{2}\frac{{R}_{\mathrm{ESR}}}{{S}^{2}}t\right)S}{3600m}$, (4)
The thickness of the DEL calculated using Eq. (4) was d_{av.wat} ~ 13.8 for water electrolyte specimens and d_{av.polym} ~ 1.0 for polymer electrolyte specimens.
Based on the calculated and experimental data (Table
Specific energy capacity of water and polymer electrolyte cells as a function of Busofit specific surface area
The obtained curves allow determining the maximum theoretical energy capacity of the cell. The results suggest that no breakthrough in energy capacity growth was made.
The dielectric permeability of this type of capacitors varies over a wide range. Such material can be for example potassium polytitanate (PPT). At a TiO_{2}/K_{2}O molar ratio of 3.7 to 6.6 this material is in the form of layered scaleshaped particles 200–800 nm in width and 10–40 in thickness. The electrical conductivity and dielectric permeability of PPT may exhibit figures typical of solid state electrolytes, semiconductors or dielectrics. The dielectric permeability of PPT may vary from 10^{3} to 10^{9} (Fig.
The effect of an increase in the dielectric permeability (ε ~ 5 · 10^{5}) due to the formation of a 10–20 nm thintunneling dielectric layer was studied.
The calculations were made for cells with an electrode material specific surface area of 1200 m^{2}/g. Figure
Study of the effect of an increase in the dielectric permeability (up to 10^{5}) due to the formation of a 10–20 nm thintunneling dielectric layer showed that the maximum theoretical specific energy capacity can reach 9 kW·h/kg for water electrolyte capacitors and 30 kW·h/kg for polymer electrolyte capacitors. These results are quite approximate since the mathematical model still has a space for improvement by taking into account the effect of electrolyte and working voltage, potentially providing for a significant breakthrough in energy capacity. One should also bear in mind that the presence of a thin dielectric layer in the DEL may increase the working voltage to far above the CCStypical figures. For example, the working voltage of aluminum capacitors may grow to 1000 V and that of tantalum ones, to 70 V.
Energy is accumulated in hybrid capacitors in the DEL and due to chemical reactions. For energy capacity calculations, LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2} additive deposited onto the cathode material of capacitors was used as a chemically active material (CAM) (Specimens 1, 2 and 3) and the parameters of that cathode material are summarized in Table
Specimen No.  Capacitor weight (g)  Specific energy capacity without CAM E_{sp} (W·h/kg)  Hybrid capacitor specific energy capacity at E_{hybr} (W·h/kg)  
CAM weight 5 % of hybrid capacitor weight  CAM weight 15 % of hybrid capacitor weight  CAM weight 35 % of hybrid capacitor weight  CAM weight 50 % of hybrid capacitor weight  
Water electrolyte cells (ε_{wat} ~ 80, d_{av.wat} ~ 13.8 nm)  
1  18  4.2  28  78  178  253 
2  25  4.9  28  78  178  253 
3  100  4.5  28  78  178  253 
Polymer electrolyte cells (ε_{polym} ~ 8, d_{av.polym} ~ 1.0 nm)  
1  20  16  49  125  276  390 
2  46  21  52  128  280  394 
3  80  12  46  122  274  388 
The energy capacity E_{CCS} is 760 W·h/kg for LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2} battery cathode and 500 W·h/kg for water electrolyte cells at 2.5 V. This allows calculating the energy capacity delivered by the CAM additive m_{CAM} which is k percents of the hybrid capacitor weight.
E_{m.} _{ CCS } = E_{CCS}m_{CAM},
where m_{CAM} = km_{hybr}.
The specific capacitor energy capacity E_{m.}_{cap} per its weight m can be calculated using the following formula:
E_{m.} _{cap} = E_{sp}m.
The specific energy capacities E_{sp} of capacitors having the weight m are summarized in Table
Since the weight of the cathode material is 50% of the cell weight, the hybrid specimen weight m_{hybr} can be calculated using the following formula:
m _{hybr} = m + m_{CCS} = m + 0.5m.
The specific energy capacity of a hybrid capacitor with CAM is calculated as follows:
${E}_{\mathrm{hybr}}=\frac{{E}_{m.\mathrm{cap}}+{E}_{m.\mathrm{CCS}}}{{m}_{\mathrm{hybr}}}$. (5)
Table
Specific energy capacity of (a) water and (b) polymer electrolyte hybrid capacitors as a function of CAM content
The calculations suggest that the energy capacity of electrochemical cells can grow to max. 350–400 W·h/kg. Further growth of the specific energy capacity requires transition to lithium/air CCS cell design and new technologies and materials. It should be noted that in that case, thinfilm technologies are also helpful for solving the task.
The dielectric permeability of this type of capacitors varies over a wide range, the energy being accumulated in the DEL and due to chemical reactions. Specific energy capacities were calculated for hybrid capacitors with thintunneling dielectric in the DEL and a dielectric permeability of 10^{3} for water electrolyte cells and 10^{2} for polymer electrolyte cells (Table
Specific energy capacities for hybrid capacitors with high dielectric permeability and CAM additions
Specimen No.  Capacitor weight (g)  Specific energy capacity without CAM E_{sp} (W·h/kg)  Hybrid capacitor specific energy capacity at E_{hybr} (W·h/kg)  
CAM weight 5 % of hybrid capacitor weight  CAM weight 15 % of hybrid capacitor weight  CAM weight 35 % of hybrid capacitor weight  CAM weight 50 % of hybrid capacitor weight  
Water electrolyte cells (ε_{wat} ~ 10^{3}, d_{av.wat} ~ 13.8 nm)  
1  18  53  60  110  210  285 
2  25  52  60  110  210  285 
3  100  65  68  118  218  293 
Polymer electrolyte cells (ε_{polym} ~ 10^{2}, d_{av.polym} ~ 1.0 nm)  
1  20  212  179  255  407  521 
2  46  299  237  313  465  579 
3  80  182  159  235  287  501 
If conventional cathode materials are used, no appreciable growth in specific energy capacity can be achieved even with new anodes (see Fig.
However, the result can be improved by using new electrolytic cell designs e.g. air/lithium CCS. The development of those electrolytic cells requires new technologies. One option is to use thinfilm technologies.
Specific energy capacities of hybrid capacitors were calculated for ZnO and LiO_{2} metal/air systems (Tables
The calculations suggest that even if a thintunneling dielectric with a relatively low dielectric permeability is used, conventional chemically active materials allow achieving CCS with a specific energy capacity of 400–500 W·h/kg, and the specific energy capacity will grow dramatically if the lithium/air system is used.
Theoretical analysis of prospective electrolytic cells suggests the necessity of developing a thinfilm technology providing for the fabrication of new generation electrode materials designed in the form of a carbon matrix with a welldeveloped surface containing a thintunneling dielectric with CAM on its surface. The main task of this technology is to provide for the deposition of nanostructured functional layers on the carbon matrix with a high specific surface area (450–500 m^{2}/g or higher). Theoretical analysis suggests that high specific energy capacities can be achieved by providing a hybrid capacitor with a thin tunneling dielectric in the DEL and CCS in which lithium or its alloys are used as CAM. Solution of this task requires a technological system providing for the fabrication of high specific surface area matrices containing a thintunneling dielectric to accommodate CAM. Thinfilm technologies are a promising development trend and therefore theoretical and physicochemical fundamentals of these technologies are being developed.
Specific energy capacity of (a) water and (b) polymer electrolyte high dielectric permeability hybrid capacitors as a function of CAM content
Specific energy capacities of hybrid capacitors with ZnO metal/air system
Specimen No.  Capacitor weight (g)  Specific energy capacity without CAM E_{sp} (W·h/kg)  Hybrid capacitor specific energy capacity at E_{hybr} (W·h/kg)  
CAM weight 5 % of hybrid capacitor weight  CAM weight 15 % of hybrid capacitor weight  CAM weight 35 % of hybrid capacitor weight  CAM weight 50 % of hybrid capacitor weight  
Water electrolyte cells (ε_{wat} ~ 10^{3}, d_{av.wat} ~ 13.8 nm)  
1  18  53  80  170  351  486 
2  25  52  80  170  351  486 
3  100  65  88  179  359  494 
Polymer electrolyte cells (ε_{polym} ~ 10^{2}, d_{av.polym} ~ 1.0 nm)  
1  20  212  186  276  457  592 
2  46  299  244  334  515  650 
3  80  182  166  256  437  572 
Specific energy capacity of (a) water and (b) polymer electrolyte hybrid capacitors with ZnO metal/air system as a function of CAM content
Specific energy capacities of hybrid capacitors with LiO_{2} metal/air system
Specimen No.  Capacitor weight (g)  Specific energy capacity without CAM E_{sp} (W·h/kg)  Hybrid capacitor specific energy capacity at E_{hybr} (W·h/kg)  
CAM weight 5 % of hybrid capacitor weight  CAM weight 15 % of hybrid capacitor weight  CAM weight 35 % of hybrid capacitor weight  CAM weight 50 % of hybrid capacitor weight  
Water electrolyte cells (ε_{wat} ~ 10^{3}, d_{av.wat} ~ 13.8 nm)  
1  18  53  492  1404  3230  4600 
2  25  52  492  1404  3230  4600 
3  100  65  500  1413  3239  4608 
Polymer electrolyte cells (ε_{polym} ~ 10^{2}, d_{av.polym} ~ 1.0 nm)  
1  20  212  682  1763  3925  5547 
2  46  299  740  1821  3983  5605 
3  80  182  662  1743  3905  5527 
This work deals with a thinfilm technology of CCS, highperformance supercapacitor systems and hybrid highperformance supercapacitor systems on the basis of a unified electrode material, which is justified by the physical and mathematical model being developed.
Study of the thinfilm technology for active layer deposition on highly developed surface carbon matrices allows reducing the internal resistance of the electrolytic cells by orders of magnitude. Since the heat emission due to current passage in the system is described by the formula
Q = I^{2}R,
the temperature decreases dramatically and hence the operation safety of the electrolytic cells increases.
Thus, a promising electrode material for hybrid capacitor structures is a nanostructured electrode material in the form of a carbon based matrix filled with nanostructured CAM.
Since an elastic matrix has a high specific surface area, energy accumulation in the electrode material occurs by two mechanisms: due to chemical reactions and in the DEL.
Carbon matrix metallization includes two stages:
First stage: magnetron vacuum titanium layer deposition on a UMRM1 roll stand type system [
Titanium was chosen due to its low weight and the possibility of eventually synthesizing sodium and potassium polytitanate. Appropriate processing of this material allows synthesizing high dielectric permeability coatings (above 10^{6}).
Second stage: coating of deeper layers and formation of target nanostructure by electropulse technologies.
The UMRM1 technological system (Fig.
The equipment was designed for operation in moderate and cold climate, location category 4 as per GOST 1515069^{1} for locations with a (22 ± 3)°C air temperature and a (60 ± 15)% relative humidity.
The equipment is powered from threephase fourwire mains with a neutral wire, 380/220 V, 50 Hz, power supply quality standard as per GOST 1310987
The equipment operation principle is titanium magnetron vacuum sputtering and titanium vapor condensation on a Busofit T40 type carbon material a tape of which is run on the winding tolls over three evaporation zones [
Figure
Figure
The metal layer on Busofit type fabric reduces the internal resistance and increases the capacity of the electrolytic cell. A more complex technical task of applying a metal layer on each thread is solved by integration of a vacuum metallization technology with an electropulse technology of nanoparticle formation in liquid media. The surface to be coated is covered with a film having a pillar structure with a highly developed surface.
To date, the minimum experimental Busofit type fabric thickness is 250–300 mm which is 45–50 thread layers stacked one upon another. It is therefore impossible to apply metallization layers on each thread (over the whole fabric depth) in vacuum. Then a liquid phase nanoparticle deposition technology is used. Figure
This task was solved by developing a liquid phase coating deposition technology for which the target fabric is completely submerged in a metal nanoparticlecontaining liquid for particle penetration to each individual thread.
Figure
One can see silver aggregation on the surface in the form of large nanoparticles and crystals which develop the fiber surface. Figure
Zinc and magnesium layers were also deposited on the carbon matrix during tests (Fig.
The electrode materials obtained were used for synthesizing highperformance supercapacitors and DEL the appearance of which is shown in Fig.
It is safe to assume that a decrease in the electrical resistance of the capacitor structure increases its specific energy capacity, and that thinfilm technologies, unlike conventional thickfilm ones, allow controlling this parameter over a wide range. The assembled capacitor structures were bedtested for residual water removal aiming at achieving a higher operation voltage. The operation voltage could be increased to 4.5–5 V, corresponding to a maximum specific energy capacity of 25–30 Q·h/kg. Theoretically, the structure in question can be operated at voltages of above 5 V for dryroom assembly, i.e., in the complete absence of moisture. Then the specific energy capacity of cells can exceed the capacity of lead batteries.
Cyclic tests of bed specimens (Fig.
Photograph of Busofit type fabric with titanium layer applied: (а) highmagnification photograph; (b) electron micrograph. Thread thickness 6.131 mm, metal layer thickness 2.052 mm
Complex metallization of Busofit type fabric: (a) vacuum titanium application and (b) silver and nickel application
(a) Specific electrical capacity and (b) electrical resistance of ESR capacitor structure as a function of electrode material contact area in cells: (1) without metallization and (2) with metallization
Prospective electrolytic energy storage cell designs were studied. We show that a significant increase in the specific energy capacity can be achieved in capacitors with thintunneling dielectric in the double electric layer and in hybrid capacitors with the DEL containing a thintunneling dielectric layer.
A thinfilm technological system was developed for the synthesis of nanostructured materials on carbon matrices with different specific surface areas allowing the manufacture of electrolytic cells and ensuring their specific energy capacity growth.
This work was carried out within State Assignment of the Ministry of Education and Science of the Russian Federation, Topic No. FSFF20230008.