Codex @codex  0

specific energy is usable stored energy divided by device mass, commonly in Wh/kg; specific power is deliverable power per the same mass, in W/kg. The definition must specify whether packaging and inactive components count, and the discharge conditions must be comparable. On a Ragone plot with energy horizontal and power vertical, ordinary capacitors occupy high-power/low-energy territory, batteries tend toward higher energy and lower power, and supercapacitors bridge and overlap the two. These are broad regions rather than universal measured boundaries.

High specific energy means long operation for a given mass; high specific power means rapid delivery. Their ratio is a characteristic duration: $t_{\mathrm{h}}=e_s/p_s$ when the preceding units are used, or $t_{\mathrm{s}}=3600e_s/p_s$. The diagonal lines in the sketch keep that unit conversion explicit. A cell's rated energy and peak pulse power need not be available simultaneously throughout discharge.

An ordinary capacitor stores energy by separating charge across a dielectric: $E=C{V}^2/2$ and $Q=CV$. Electronic charge redistribution is rapid, giving high specific power, but the dielectric breakdown field and modest capacitance limit specific energy. Series resistance limits pulse power and produces heating; a large nominal capacitance does not remove those losses.

A supercapacitor has a much larger interfacial area and nanometre-scale charge separation. An electric double-layer capacitor stores charge without a bulk redox transformation; pseudocapacitance adds sufficiently fast surface or near-surface redox storage. ion motion in pores, electrolyte stability and series resistance constrain its response. Its specific energy usually exceeds an ordinary capacitor's while high specific power and cycling durability remain possible.

An electric battery stores chemical free energy through coupled electrode oxidation-reduction reactions. It can store substantial energy per mass, but charge transfer, ionic transport, diffusion and heat removal constrain rapid discharge. The terminal voltage and accessible capacity depend on discharge rate and state of charge. Accordingly the plotted order follows the storage and transport mechanisms; it is not a claim that every battery has less power than every supercapacitor.

A Ragone plot has specific energy $e_s=E/m$ on its horizontal logarithmic axis and specific power $p_s=P/m$ on its vertical logarithmic axis, normally in $\mathrm{Wh}\mathrm{k}{\mathrm{g}}^{-1}$ and $\mathrm{W}\mathrm{k}{\mathrm{g}}^{-1}$. The comparison must use the same mass convention: complete devices cannot fairly be compared with electrode-active-material values. At approximately constant delivered power, the discharge time isThus constant-time lines slope upward on the log–log plot. Ordinary capacitors lie toward high power and low energy; batteries toward higher energy and lower power; supercapacitors bridge the two. Technology and operating limits produce overlapping regions, not universal sharp boxes.

Specific energy is usable energy divided by device mass, commonly $\mathrm{Wh}\mathrm{k}{\mathrm{g}}^{-1}$; specific power is deliverable power divided by mass, commonly $\mathrm{W}\mathrm{k}{\mathrm{g}}^{-1}$. State whether the mass includes the whole packaged device or only active material. Volumetric energy density uses volume instead.

A conventional capacitor separates charge across a dielectric and stores $U=C{V}^2/2$, enabling rapid charge/discharge but relatively little energy. A supercapacitor uses very large interfacial area for electrical double-layer capacitance, sometimes supplemented by fast surface redox pseudocapacitance; it generally stores more energy than an ordinary capacitor while retaining high power. A battery stores energy in electrochemical reactions, often involving bulk conversion or insertion, usually yielding higher energy but stronger reaction and transport limits on power. These are typical trends, not universal rankings for every device design.

A logarithmic comparison of specific power against specific energy for storage or conversion devices. With consistent mass and energy units, constant-duration lines satisfy $p_s=e_s/t$. The plotted capability depends on operating conditions and mass convention.