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Energy engineering has developed alternative methods for storing electricity that are technically simpler and less powerful than traditional pumped hydroelectric power stations
Currently, the primary focus is on electrochemical technologies, which convert electrical energy into chemical energy stored within substances. These systems operate based on the interaction between two electrodes and a special fluid known as the electrolyte. Recently, research has expanded to include systems utilizing not only liquid electrolytes but also solid-state electrolytes. This fundamental principle underlies the electrochemical batteries already widely in use today.
Electrochemical energy storage is one of the most commonly employed technologies in both industry and everyday life. The key principle behind all rechargeable batteries is the reversibility of the chemical reactions, allowing for repeated cycling and reuse.
A particularly interesting solution is the flow battery, which uses liquid electrolytes stored in separate tanks and separated by a membrane.
These systems can increase capacity by continuously pumping electrolytes through the electrochemical cell, generating energy during the reaction.
The most commercially common types of rechargeable batteries include lead-acid, lithium-ion, and nickel-cadmium batteries.
In lead-acid batteries, lead dioxide (PbO₂) and metallic lead (Pb) serve as the active materials, while a sulfuric acid solution acts as the electrolyte.
During charging and discharging, electrochemical redox (oxidation-reduction) reactions take place at the electrodes, with the electrolyte facilitating ion transfer between them. Ion concentrations either decrease or new ions form as part of this process.
This is how electrical energy is stored during charging and released during discharging. At the negative electrode, liquid-phase processes occur following a "dissolution-precipitation" mechanism. The overall rate of charging and discharging is influenced by heterogeneous non-electrochemical reactions—such as crystallization and dissolution—along with ion diffusion.
Lead-acid batteries are widely used due to their reliability and cost-effectiveness, but they have significant drawbacks. These include a relatively low specific energy density (typically 10–30 Wh/kg), the use of toxic lead, a limited number of charge/discharge cycles, and a low allowable depth of discharge.
Depending on their application, lead-acid batteries are classified into the following categories:
• Starter batteries – used to start internal combustion engines;
• Stationary batteries – serve as backup power sources, including in renewable energy systems;
• Traction batteries – designed for use in electric transport vehicles;
• Portable batteries – used to power tools and electronic devices.
Lithium-ion batteries use a carbon-based material as the negative electrode (anode), into which lithium ions can be reversibly intercalated (inserted).
The positive electrode (cathode) is typically made of lithium cobalt oxide (LiCoO₂), which also allows for reversible intercalation of lithium ions.
The operating principle of this electrochemical system is based on intercalation — the reversible insertion of ions or molecules between the layers of a host material. Lithium ions form different compounds depending on the electrochemical potential at each electrode.
The movement of lithium ions between the electrodes is enabled by an organic electrolyte, usually consisting of a blend of organic solvents and a lithium salt.
Compared to traditional acid- or alkaline-based systems, the use of organic electrolytes allows for higher voltage output.
During charging, lithium ions are inserted into the anode material. During discharge, these ions migrate toward the cathode, while the freed electrons flow through the external circuit, generating electric current.
Lithium-ion batteries are known for their high energy capacity and deep charge/discharge cycles, typically allowing for a depth of discharge (DoD) of 70–80%. However, their cost-effectiveness depends on the specific electrochemical composition of the electrodes, operating temperatures, and usage conditions.
Drawbacks of lithium-ion batteries include:
• High production cost
• Sloped (nonlinear) discharge voltage curve
• Relatively high self-discharge rate
Due to their high specific energy, the production of lithium-ion systems has surged significantly in recent years.
The latest advancement in electrical energy storage—representing the third generation of nickel-ion battery technology — involves systems that use lithium iron phosphate (LiFePO₄) as the cathode material.
Lithium iron phosphate is an excellent battery material, capable of releasing nearly all stored lithium while maintaining chemical stability. At the same time, it retains the key advantage of lithium-ion batteries: high specific energy density.
As a result, third-generation lithium-ion batteries have become both safe and highly efficient.
Nickel-cadmium batteries have been known and used for many years. Their operating principle is based on the formation of cadmium hydroxide at the anode and nickel hydroxide at the cathode. The electrolyte is a potassium hydroxide solution, which is why these batteries are also referred to as alkaline batteries. Ni-Cd batteries are capable of functioning at low temperatures and can handle much higher charge and discharge currents compared to lead-acid batteries.
These advantages make nickel-cadmium batteries well-suited for transportation, aviation, and stationary power systems. However, they do have a notable drawback known as the “memory effect” — a phenomenon where the battery’s energy capacity significantly decreases if it is repeatedly charged or discharged without completing the full cycle. To counter this, special charging algorithms are used.
Despite the previously mentioned disadvantages, nickel-cadmium (Ni-Cd) batteries were once considered a viable alternative to lead-acid batteries in electric transport — at least until the emergence of more modern and lower-maintenance systems.
However, Ni-Cd batteries never fully replaced lead-acid batteries, primarily due:
• High production costs
• Labor-intensive manufacturing processes
• The limited availability of cadmium and nickel